I bought the Trilogy on the Wii and loved all three games. But here's a piece of advice for newcomers: I love Echoes (it might be my favorite), but I recommend playing it on Normal first, not Veteran.
I know Normal difficulty for Echoes is actually a new, easier mode than the Normal setting for Echoes on the GC, but... well, it isn't only "easier"; it's better. The thing is, you have to shoot a lot in order to kill your enemies in Echoes, and since the game is pretty long, it gets really, really boring. I was about 70% complete and couldn't stand the game anymore; then I quit and restarted the game on easy. The game is so much better this way, trust me.
For the other two games, I recommend playing on Veteran.
Yeah. The ammo-sponge enemies are one of the gripes I have with the game. You can kill many enemies more quickly with the right beam, but even then we are talking multiple charge shots, compared to Prime 1 where only a handful of enemies required that kind of a beating (and the plasma beam tears through
everything). I feel like the shooting isn't really supposed to be the emphasis, but Prime 2 puts a bit too much on it.
I kind of get the idea behind the ammo system, but I don't really feel like it adds anything to the game. Ammo isn't really difficult to get (if it's really low, one crate/cache will give you a bunch) so it mostly ends up being a mental factor unless you just ignore the ammo counter, which is doable as long as you don't end up wasting a bunch of it.
And the dark world is drab as hell. The taking damage thing doesn't bother me as much as it did when I first played it, but the overall look of it is just really boring. It wouldn't be bad, but it all looks the same regardless of which part of the dark world you are in, and you go in and out regularly.
Then there is the key hunt. Meh. It's a "fault" of every game in the series but Echoes has it at its worst, because only 2-3 of the keys can be gotten before you get some near-end-game equipment (read: light suit). And even those you can get sooner often require you to know where they are in advance, since they are undetectable until you get... some near-end-game equipment.