Summing up Diablo 3 in 1 sentence:

Really high production value, but a quarter of the fun.

Basically, par the course for this gemneration of games.

While it's hard to speak about how I will feel in the long run since I did play Diablo II for years but right now I would say I'm enjoying Diablo III as much as I did Diablo II when it came out. I'm also really happy because this time around the melee classes are actually interesting to me while I couldn't get into the Barb and Paladin in Diablo II.

You obviously didn't play the assassin or the druid.

Yes I did, Trap Assassin was fun, I always got bored of shape-shifting Druids.

Now, it isn't fair the compare a game that has only been around for 3 weeks and no major updates so far to another one's expansion and that already had 9 full fledged patches as well.

Diablo II's initial experience was also full of flaws on the online component, which was not as visible because of both a smaller launch (compared to D3) and the fact that most people back then didn't have a proper internet to handle an online experience such as D2 (remember how much pain the Lagomancers caused). Plus, while the SP experience was far superior to DIII's (which isn't hard to do), it too had some major flaws as well. 

Not defending DIII, because they've done a lot of things bad on DIII that have no defense whatsover, but these kind of comparisons have to be made on equal grounds.

Almost finished with Act 2 Inferno.
I'm up to the Archives ( 2 more dungeons and then 2 more bosses..)
Things were not so bad until I got to Cave of the Betrayer level 1 and then I hit a brick wall...
There's just no way I can deal with Spider Broodling rares or champions in this tunnel.
After 15 reloads and 100 of deaths I ended up parking the elites in a side a room and skipping them.

Vault of the Assassin on the other hand was surprisingly smooth ( and had at least a dozen elites packs).

Right now my wizard is running 32k hp, 20.4k dps, 6.2k armor and ~420 resists with buffs on.
Nothing can one hit me but I still die a lot to any elite fast ( whether they are have the mods or are of a kind that is naturally fast). I will die repeatly if i encounter elites at the start of a zone without having been able to clear a safe area...

Other things of note :
Most invisble snake elites packs destroy me( heck I died like 5 times rescuing Leah..) but the snake magus elites are surprisingly easy ( they stand in their damage field and let you nuke them all night long).

I have not made any cash in Act 2 due to high repairs costs and some jewelry upgrades but I found a couple of good items, one of them an off hand WD that I was able to sell for 1 million gold and that funded  some of my upgrades...

If I can beat Belial ( not sure about that) I might actually go back and farm Vault of the Assassin if Act 3 is too hard...

I'm seriously thinking replacing Diamond Skin with teleport to stop dying so much to waller/jailers..

Invulnerable minions have actually not proven to be so bad ( I run arcane orb + celestial rune + venom hydra so my spells hits everything not just the first mob behind me) but anything fast or vortex + knockback is just death.. Wallers and fire chains is death too in tight corridors...

D3 had indeed many flaws but lets not forget that D2 wasn't that polished at release either.

1 week after D2 release I was solo clearing Hell Act 4 in 8 players mode with my firewall sorcerer and had beat all the challenge the game could provide ( and it's not like skills was required either, fill the screen with firewalls, wait a couple secs, loot, rinse and repeat)....

Until Lord of Destruction came out there hadn't been a single time in D2 where I felt I would actually be better playing co-op than playing single player......

Until very late in its release cycle, D2 was a game where you spent your time farming for items that you actually did not need to beat the game, it was more like a collector thingy, I have all these nice items, but I don't really need them to beat anything the game can throw at me....( the whole point was get better gear so I can farm faster and get even more gear...which I didn't need to beat any boss..)

People have to stop comparing a freshly released game with a game that had years of polish, patches and expansions...

I am not looking at the flaws that will be fixed with time. I am looking at the more core gameplay. There was a certain satisfactoin in mowing through 3 screens' worth of enemies, even if you only found junk, over killing half a screen, spend the next 2 mins killing something with a colored name, find a bunch of "useful" things which turned out not to be good to beign with. The pacing is the problem with this game.

Hmmm, I really didn't have that feeling early on in Diablo 2. Remember that it was not until patch 1.05 that they upped item quantity a lot (mostly affixes), so most of your time was really just filled with mowing down mobs for nothing (which was goddamn awful on Act IV if you were a melee class....Doom Knight *shudders*). And let's not forget that D2 LOD more than doubled the item quantities as well, given that there was no Elite tier inventory, charms and whatnot. 

I don't have many problems with the pacing of D3 up until Inferno. While it's much easier than D2 (well, at least if you didn't play D2 online, in which case, it was also easy as D3), I still find a lot of satisfaction going up agaisnt a group of mobs to see what lot they drop.

You know, I haven't played D2 in so long I completely forgot the fact that charms existed. And now I have something else to not like about D3.