Yes, Trek makes nice bikes, they also make the gary Fisher brand. Both hold up very well. If your just getting into the sport, that bike will suit you just fine. It has entry level components, but if you really get into the sport, it can be upgraded with nicer parts down the road. Looks like Trek has the bike listed at retail for 599.99, so if the price you listed is for a brand new bike, its not a steal, but your not getting ripped off either.
I looked at bikes for a while, the 4300 was a bike I considered for a long time. I ultimately chose the Specialized Rockhopper. However in what I learned and when I test rode the bike I knew the 4300 felt nice and has a decent price point.
It's a nice bike. Again, it's not a super deal, but it's fair. I'de look at the Rockhpper "base model" first ($520 at my LBS). Then the Fisher Wahoo ($539 at my LBS.) Fisher has the G2 geometry, longer top tube. It's more of a leaned over feel, a bit more than I like (I have an older fisher marlin/wahoo).
But if it fits, and you like it, then it's worth it.
I personally just bought this bike, as I have decided to get back into biking since coming to NY for school. The three bikes I was considering were the 4300D, the 6000D, and Gary Fisher Marlin. I ended up with a 4300D as most of the bike is great for the money, and soon I will be replacing the fork to something a little more in line with the type of riding I will be doing.
Only downside is the rotors, they squeal when they are wet, I know this is a really common problem with discs, but they are louder then the Fuel 97 I had before coming to school.