The overwatch rifle was very annoying. Yes, it was quite powerful, but it still took most of a magazine to kill a Combine soldier at anything other than close range (if only because it was so goddamned innaccurate), and you never had enough ammo to use it for prolonged periods (with it burning through a whole clip in a few seconds, and you carrying 2 spares). I got really annoyed at that - you couldn't carry enough ammo for most weapons. The rocket launcher was a case in point - you only carried 3 spare rounds. That's not even enough to kill ONE gunship! I spent more time looking for rockets than I did firing the thing...
I liked the shotgun and the magnum - they were pretty useful, though again the inability to carry enough ammo meant that you couldn't rely on them for a protracted engagement. The gravity gun was great, though I wish it had a longer "push" range, something more like the "pull" range.
I actually loved playing the game - the environments were stunning, the sound was fantastic, and the graphics and lighting was sublime (much better than Doom 3 IMO). Playing it was great fun, and the characters were all pretty well defined (apart from the priest bloke in Ravenholm).
The thing I hated about it was the complete lack of major plot development, the limited number of bad guys (and the fact that you'd seen them all by about halfway through the game), the lack of alien enemies (besides the pain-in-the-arse headcrab/zombie mutations), and the fact that everything was underused. Dog, for example, was brilliant. I'd have loved to have played through a few levels with him, but he gets used only very briefly, in order to move heavy things for you.
The ending was very irritating, if only because it leaves you on such a cliffhanger, and tells you nothing about anything very much. I'm now half-expecting there to be a further sequel within the next year or so, like Kill Bill parts 1 and 2, to actually provide some storyline (obviously, I hope it's better than KB2, which was dreadful). As-is, it feels like a stopgap that was released in order to satisfy baying fans after the whole hacking thing went on. I keep thinking that Valve must have saved up all the really tasty stuff for later, or they've played a cruel trick on us all...