Wishbone: It's a very hard game to define, as it mixes up different genres and switches between protagonists. Basically, it's a 1st/3rd person shooter, with occasional RTS elements. But where Giants really shines is its humor. It will have you laughing out loud again and again. Here's a small sample:
http://www.youtube.com/watch?v=1zu7u9AcvHc orcishgamer: The Mecs get jetpacks, 'nuff said. Play Giants. Make sure you path out the green blood (back to red) and Delphi's clothing change (publisher forced a bikini at the last moment, it's a separate texture so you can just rename it and you get the game as intended).
I bought Giants back when it was new and could barely run on my PC, and then I RE-bought it for PS2. I've regretted a lot of discount purchases, like when I bought the new Alien vs. Predator off of Steam for $8, but I have never regretted spending $80+ on Giants. If GOG is offering it for $6, BUY THE HELL OUT OF IT!
Oh, and while you're at it, buy
Sacrifice. It's an RTS from the same team that created Earthworm Jim, but you actually play as a wizard who's down on the battlefield with your troops, so you can summon reinforcements and support spells (and by "support," I mean tornadoes, volcanoes, and even Death himself). It also comes with a map editor so you can create your own custom campaigns/multiplayer scenarios just like you could in Star Craft.