Putting in my two cents as well.
Black and White were absolutely terrible. I clawed and dragged myself through my Black version, just so I could get to the end with the hope that maybe maybe it would get better or redeem itself.
Spoiler alert: it didn't.
The storyline was okay, but there was so, sooo much more that could've been done with the evil team. Their stated goals didn't really align with many of their member's attitudes, and the real leader ended up... well... not to give anything away, but it was really cheesy, stereotypical, and last minute.
If you like any of the old Pokemon, don't expect to see them in Black and White. You won't. Not until the postgame, which has absolutely nothing in it. I'm serious. There's nothing to do at all.
I played Black when it was the newest game in the series, and man, was I disappointed. I was so disgusted with Black that I was ready to be rid of Pokemon. Back when they WERE Gen 5, I thought that if this is the direction Game Freak is going with the series, than I wanted no part of it.
Black2 and White2 came out sometime later, and I only bought Black2 as a way to mercifully end my Unova adventure, like you would mercifully end the life of an animal so old that all it can do is pee on itself and ruin the house, and say goodbye to the sinking ship that was Pokemon.
I was so, so happy with what I found in Black2. Really, the difference is night and day. The storyline is superb for a Pokemon game (though not perfect, no Pokemon story really is, so I let that pass), the characters were fun and believable, and they fixed literally EVERY gripe I had with the original B/W. The postgame was plentiful enough that it kept you busy, and the Pokemon World Tournament was loads of fun. If you like the older Pokemon, you can get them again. The added Join Avenue was super cool, but now that X/Y is out, I'm not sure if you'll be able to expand it as much as we did back in the day, because it was a cooperative effort to grow it by trading and battling. The region is huge and expansive, and you'll find yourself marveling at how big it is. B2/W2 Unova is easily the biggest region we've see so far. I'm not sure if I could name any one Pokemon game as my favorite, but damn, Black2 really blew many of the games in the series out of the water for me. It was that good.
If you're used to X/Y, or even Gen 4, you'll have a few things to adjust to for Gen 5. Pretty much nothing is in 3D, which is to be expected. The Pokemon move constantly in battle, but can look a little distorted because of it. There is no Fairy-type, as that is a Gen 6 thing, so the type chart is the same as you remember it. AND Gen 5 is backwards compatible with Gen 4, and Gen 5 games (B/W/B2/W2) are the only ones that are compatible with the Pokemon Bank's Poke Transporter, so if you want to bring your Gen 4 Pokemon up into X/Y, they'll need to go through a Gen 5 game first.
That said, if you're only in the money to buy one game from Gen 5, pick either Black2 or White2. Hands down. Read the first game's story online before playing them, and you'll be all caught up. If you want both games, then buy either Black and Black2, OR White and White2. The version exclusives and legendaries are set up in such a way that you want the same color for both the first and second games so you don't have overlap. You will suffer going through the first ones. You will. But you'll have a better grasp on the story and the newer Pokemon before you head into B2/W2, where you'll breathe a sigh of relief and put down the noose you were weaving at the end of B/W when the big bad... well, I'll let you figure it out.
