I've actually dabbled with all of the Wario Land games in the past, but that was quite a while ago and all I really remember is that I thought they were good games. I spent the last three nights playing through Wario Land II via the 3DS Virtual Console, and I had a really good time with this one.
First off - it looks GREAT. I often think that the GBC is written off as just a colorized Game Boy, but in reality there's some truly excellent looking graphics on GBC and the color pallet is impressive when used properly. This game is a solid example of that.
I never really liked Wario as a character. He's just kind of goofy and bullish. I don't know. He's certainly not as elegant to control as Mario. But honestly, all of that kind of goes away when playing a game this good.
The whole conceit of Wario Land II - and its sequels - is that Wario can't die. This makes the games way more about puzzle solving than pure platforming. Call me crazy, but I tend to prefer this approach more than in the original Wario Land (AKA: Super Mario Land 3. Ugh. I hate when they can't just decide on a single title...). So like enemies can attack Wario and instead of hurting him it turns him into SOMETHING. Like there's a zombie Wario or a drunk Wario or a Wario on fire and so on, and all of these altered states can be used to your advantage to solve various puzzles. It's good stuff.
The game is a decent length. It probably took me like four hours to run through it on a blind first attempt. Apparently there's lots of alternate paths through the game, as well as a bunch of hidden levels and stuff. So that's cool. It means there's some decent replayability here. I don't really care much about collecting all the treasure or whatever.
But yeah. Definitely a very good game. One I'd play again sometime.