I just beat it. I have mixed feelings about it.
The gameplay is really good, I especially loved the early acts, it was a nice change from the usual fare. The addition of Drebin's shop made killing enemies fun, and customizing weapons was a nice touch too. Technically, this is the ultimate Metal Gear game. The gameplay-to-cutscene ratio was also better in the first few acts, it's kinda nice how this game favors cutscenes to codec calls, compared to MGS2 and 3. The gameplay keeps changing throughout the game, and it prevents it from getting boring (REX vs. RAY... wow!)
Now for the story... uh... I'll admit I'm a fan of the series, and that I love how ridiculous/bad-written/weird/convoluted the plot is in every game. I'm aware it's bad, but it's also funny, it's probably the high quality of the whole thing, and the little things Kojima likes to throw it. This game... it was too much at times. It's like some bad fanfiction trying hard to tie all the loose ends while throwing in every single character in the series for fanservice. The story is just an extension, other games could be played on their own, but this one depends too much on the other games. The part after the voice actors credits was really bad, it's just the worst way that scene could've been handled, and I hated how that guy keeps talking and won't die (of course, bringing him back in the first place was dumb, but still...)
With that said, the story has its highlights too. I enjoyed the action scenes, and the interactivity in some of them. The bosses were interesting too, not so much for their back stories (at least they have stories, unlike the ones in MGS3), but because of the neat ideas the battles had. I guess it's what you'd expect from the series after all.
I played some online games and I thought they were fun. I only played death match and sneaking mission, but I liked how well the new game mechanics are extended online. I haven't played the online expansion of MGS3 after all.
Overall it was a very good game. The story was really ridiculous (even for metal gear solid games), some of the cutscenes were too awkward and boring, and the whole nostalgia/fanservice thing was overdone. But it still had great gameplay, graphics, music, and atmosphere. I don't regret getting a PS3 for it, but I'd have really liked the game to be longer (I beat it in 20 hours, half of which are probably cutscenes). It would've nice if they had a battlefield part somewhere in the later acts (maybe in Europe?), both to extend gameplay and to balance the amount of cutscenes.
In retrospect, MGS1 is still the best game in the series. I'm not sure which is the next best game though, they all had their ups and downs; I guess MGS4 would be at the top when ranked by gameplay, and at the bottom when ranked by story.