I ride to Uni every day, about 10 kilometres each way with some nasty hills in between. I own a Giant Rincon 21-speed mountain bike which is about ten-years old now and is still a fantastic bike. I've had to do a lot to it recently, replacing the tyres and the gears, chain, brakes and such, to keep it in shape, but it has lasted a very long time and still holds its own.
I can overtake those pompous idiots on $4000 road-racers, and they tend to look a bit sheepish when they're passed by a mountain bike on an open road, especially when I'm laden down with my back-pack and all.
I've found the Giant's to be fantastic bikes. Mine is tough enough to be able to handle a breakneck-downhill run every night along a potholed road with utmost confidence. I wouldn't go out without my BLT rechargable light-kit, which lasts 3 hours and is bright enough to startle traffic. Also, I recommend you get quality, well-matched mud-guards. While they may detract from the "coolness" of the bike, there is nothing cool about having a big muddy stripe up your back. Add on a packrack for practicality and a quality lock-cable, and you've got a bike that's easy to live with.