While I can't speak from personal experience on the "big box store" Diamond Backs and such-and I have owned and built from the frame up more than a couple $2,000-3,500 bikes over the years,my current mtn bike came from bikesdirect.com and was only $350 shipped (came as a rigid aluminum singlespeed),though I did swap out most of the components for higher end bits I had on hand (I'm currently-by that I mean it's all torn apart and hanging in the repair stand,like,as we speak) replacing the aluminum frame with a sweet On One INbred steel frame I lucked into scoring,but there was nothing wrong with the Gravity frame (or any of it's stock components,for that matter,cheap honest SS riding).
1) For the kind of riding you mentioned that you do,those Diamond Backs should last you years and thousands of fun miles,my friend,as would many of Mike's bikes at bikesdirect.com (who are actualyl a pretty good group to deal with from my personal experiences of owning 2 of their mtn bikes).
2)Don't worry about what "elitist bikers" say,they ain't gunna buy or maintain your ride for you,and they don't pay any of your bills. Ride what you can afford that fits you well and that you can budget (including you "$100" bike).
3)If you get into more technical riding,the frame on those should be well worth upgrading a few key components as budget,time and skill allow (and for me,that's half the fun anyways
),or just ride the wheels off it. Almost everyone wants a Ferrari (or similar supercar),but most everyone gets their driving enjoyment out of their mor epractical cars/trucks/SUV's/minivans that cost a fraction of what the thoroughbred did
