100% or 100 point scale? A scale of 1 to 10? Five stars? Tier from F to S? What does your system mean for you? I'm particularly interested in how people use a 100 point scale to conclude at a score like 89 out of 100 instead of rounding to a 9.

I use a scale of 1 to 10.

1. Absolutely broken or unplayable
2. Extremly bad in most aspects, a pain to play
3. Bad in most aspects, very unpleasent
4. Poor in most aspects, unfun
5. Average/Bland/bits of fleeting fun
6. Average/poor/good in few aspects with some mild fun to be had
7. Good in most aspects and fun
8. Great in most aspects with loads of fun
9. Phenomenal in most aspects with immense fun
10. Masterpiece and immensely addicting, near flawless

Or close enough to the above, technicals like image stability and other objective stuff make sure that some games get stuck between these classifications as points are reduced accordingly and I have to judge wheter to round up or down. 

There is no 0 on a 1-point scale in my case. Even the biggest pieces of garbage get a 1 or 0.5 on a 20-point scale.
1. Abysmal, more or less unplayable usually in function but at least in enjoyment.
2. Terrible, it's really really bad.
3. Bad. It can be functional, but no enjoyment worth mentioning.
4. Poor. Little bright spots of enjoyment sometimes, but a poorly made game.
5. Mediocre. More or less equally good and bad. It's probably not worth playing, especially if it's not a short game.
6. Okay. More good than bad but not an outright recommendation.
7. Good/decent. The lowest end of a recommendation (as a 6 usually isn't worth more than a tepid recommendation if any at all) with more good than bad.
8. Very good. Not a must-play for the majority, but really fun and often very well made. Recommended
9. Excellent. Must-play for most. This game is overwhelming full of positives and a great time.
10. The Best. Must-play, full-stop. The experience you get from this game is near-perfect.

I don't like numbered or letter grades. They oversimplify things. People only look at a number and make a blanket judgment based on a Metacritic page. I have to personally look into many aspects.

Lettered grades especially seem like an issue. What's an F? Anything below 6 on a 10 point scale? Not until below 5? While you have to look at the words of a review as well, an F just seems to broadly translate to not recommending a title ranging from the worst game ever to a meh game. 

5 stars with half-stars is probably the most meaningful scale to me. A 88, 91, and 93 could probably all be roughly equal to me, depending on the games, so such a fine-grained scale doesn't really provide much extra value over a coarser scale. On the other hand, a scale from 0 to 10 is heavily biased towards the upper end, which I don't feel like is the case with a 5-star scale, with or without half-stars: 3 stars is roughly in the middle, just as 50 points is, but whereas I would call a 3-star game pretty average, I'd certainly call a 50-point game trash. The equal of a 3-star game is actually something like a 70-point game, or maybe even more, because I would expect most 3-star games to be pretty passable, but most 70-point games are probably getting into the risky territory already. Also, I feel like using stars instead of precise points puts more focus on the review itself, which better acknowledges the complexities of rating a game.

Of course I don't really rate games a lot, so who cares, but if I did, I'd probably use a 5-star scale with half-stars.

Great game
Good game
Bad game.

I think it's silly that people have one all-encompassing rating for a game that takes into consideration both its value as a work of art, and how well it is (or isn't) technically realized. So many games have come and gone where they have great stories/music/atmosphere/setting/etc and even great gameplay in theory, but are lambasted for having glitches or a poorly optimized port. I understand that those issues are relevant and stop the player from immersing themselves into what the game is supposed to be, but technical issues are a completely separate problem.

What I'm saying is: You so often see people rating a very interesting game a 0 or 1 out of 10 because it came out with many glitches that are likely to be patched in the future, and then rate the blandest and most boring stuff a 6 out of 10 because even if it's uninteresting, it's functional. The scale is not being used well. "Functional" should not guarantee you a decent score.

I always thought games should have two scores. One for what they are as a creative work, and one for the technical aspects of it. That way it's also easier to rate games across several platforms: each platform gets its own technical rating for the quality of the port, but there's only one creative rating throughout. And while the technical rating could change over time with patches, the creative rating really shouldn't - with an exception for games with big DLCs that change the experience fundamentally.

I usually define them as: One of my favourite games of all time, I really love this one, I like this game, or this game is not for me. There's no real score, no real reason, it's just how I feel about them. For me, the positives outweigh the negatives, and the negatives only really mean anything serious if I don't care much about the positives.

Some of my most disliked mechanics exist in or originated in my favourite games. For example, Xenogears is my favourite game of all time, it is also the birth of one of my least favourite mechanics of all time - battle combos, that is combining multiple attacks when a single attack would suffice.
Final Fantasy 6 is another one of my favourite games of all time, it was also the birth of mini-game battle inputs, another thing I loath in RPGs.
One of my least favourite things in any type of game is dungeon crawling, yet I love NES's Metroid, and that game is one big dungeon crawl.
Shallow Mary Sue characters are also a giant turn-off for me, but Skies of Arcadia somehow won me over.

5 star system, with half stars. I picked that up long, long ago on Adventure Gamers, site dedicated to adventure games that's been around since mid 90s, since it gives broad tiers with half steps for some variance. So there definitions:

5 stars - Classic
An instant, hall of fame classic. Although not "perfect", we award our top score only to those games that set the highest standard for quality.

4.5 stars - Excellent
A superb game that excels in just about every area, held back only by one or two notable flaws or a collection of smaller ones that prevent the game from earning full marks.

4 stars - Very good
A game of very high quality. Although some aspects might have been executed better, we would recommend this game without hesitation.

3.5 stars - Good
A solid adventure that is generally enjoyable, though it lacks enough polish, consistency or ambition to recommend without caution.

3 stars - Decent
A qualified success; the positive aspects still outnumber the negative, but the weaknesses noticeably hinder the experience.

2.5 stars - Uneven
A near-equal balance of good and bad that can make a game either fall disappointingly short of its evident potential or be mildly entertaining despite its many failings. 

2 stars - Underwhelming
There are some good elements that shine through occasionally, but generally the game is not a positive experience.

1.5 stars - Poor
Shows just enough promise to hold your attention, but is buried under too many problems to be enjoyable.

1 star - Awful
Offers a tiny glimpse of what might have been, but manages to execute just about everything wrong.

0.5 stars - Unplayable
A complete waste of time with no redeeming qualities whatsoever. We would not recommend this game to anyone, at any price. Avoid.

I do use X/10 with halfsteps as well for a bit finer steps, but 7/10 just doesn't feel as right to me as 3.5/5 stars.

That's why I liked that, back in days, review scores had different categories being scored as well as overall score.