Has any legendary Pokemon impressed you or disappointed you once you saw/used it? If so, in what way?
Hmm, I've not really been one to actually use legendaries that much, so it hasn't left much room to be surprised by how good a legendary actually is, since I've mainly just used the ones I knew were good, but I can think of two examples of this. First one was with Lugia, which I believed I mentioned in my sign-up post. Basically, the legendaries of the second generation took years for me to actually get, since I couldn't find Lugia, which was the title legendary of my game, all the legendary beasts would be random encounters, and I had really bad luck with finding those, and I could never figure out how to get past the Snorlax blocking Diglett Cave, and every time someone helped me get past it, I always restarted the game before I could get the Rainbow Wing. 5 year old me everybody! But yeah, first time I ever even saw Lugia, was when my brother was battling one of his friends who had a Lugia, and it totally wrecked his team. I remember he even traded all of the legendaries over from our Blue version, and he still managed to get his butt whooped :p But yeah, I was really surprised with how strong Lugia was, and really wanted it for myself, but unfortunately I didn't really have the patience to check through all of Whirl Isles, and thus it took quite some time before I found. But I eventually did, and it truly lived up to my expectations, except my brother had begun his quest in catching 'em all and leveling them all up to 100 at that point (he got extremely far, then lost his save file due to the battery running out :(), so I had no chance of beating him ;D But yeah, that was the first one that genuinely surprised me, simply because of how it wrecked my brother's team, which I was sure was the best :O
Second one would be all of the legendary birds in my playthrough of Leaf Green, especially Moltres. Basically, aside from Articuno, I didn't really used the birds that much in the first generation games, because they weren't really that strong, and specifically Moltres, because the moves it learned were terrible. I believe the only Fire type move it had when you caught it was Fire Spin, and that did nothing to help me in the E4, so I guess you could say I was really disappointed with that one >:[ But yeah, its moveset was improved a notch, and I actually used it in Leaf Green. Zapdos was pretty surprising as well, just because of how strong it was. I can't remember of it was the same way in the first generation games, but I definitely wasn't prepared for how much damage its Drill Peck would do.
I haven't really played any of the other generation games quite as much, since I was quick to lose my Ruby, and never owned a fourth generation or fifth generation game for myself, aside from Black 2, which had nothing to really surprise me in any way :p