While I will often try and get achievements in a game I buy I would NEVER EVER buy/rent a game purely for achievements. That just seems like a waste.
Personally though I always look at % completion not score to see how good someone is at games. For example a friend of mine has a gamerscore of around 30.5k compared to my 14.35K. However my score is out of a possible 17450. Whereas his is out of 52405. I mean anyone can have a high gamerscore, just play a huge amount of games. Having a high % is really difficult. There are some games where I will never ever make 100% like DoA4. And funnily you mention Kameo as an easy achievement game. You can get 600 incredibly easily, 650 easy, 700easy, 1000 insanely tough.
I mean if someone has 1000 in TMNT and another person has 800 in DoA4 then I know who the better gamer is.
I do really love achievements though. The fact I can just look at someones profile and get a good idea of what type of gamer they are is great. A lot of games but low scores in them? Probably a hardcore gamer who either isn't very good or gets bored quickly. Few games with low scores, a casual gamer. High scores in lots of shooters, good twitch player etc etc. 1000/1000 in DoA:XBV2? Probably has a sore wrist.