Here's my opinion on those bikes, I had a bike like that similar model the XR 150 a long time ago, and yes in comparison to the bike I have now, it sucks, really bad. The bike was heavy, not to mention the suspension is not very efficient at all, because when your pedaling your going to notice a lot of pedal bob where as the more expensive bikes that you can get will have more advanced suspension designs or better rear shocks that they will not have that much of a problem.
To answer your questions:
1. In my opinion I would not ride them on trails, they wouldn't be suited for it, reliability and durability of parts are questionable. When I had my XR 150 I was using it for commuting to and from school and the worst terrain I went over was rough flat surfaces.
2. Having a seat that's to soft or wide isn't good for trail use because it restricts movement, it's good for commuter bikes but not for trails.
3. You can expect to attain higher speeds from a bike obtained from an LBS since there will be a lot less power loss do to better suspension, bike fit, and lighter weight.
4. Again the bike you get at your LBS will be better, do to reasons stated in previous answer.
5. Dept store full suspension bikes were never meant to be taken out on trails. Not sure what you mean by that last part though.
To answer your questions:
1. In my opinion I would not ride them on trails, they wouldn't be suited for it, reliability and durability of parts are questionable. When I had my XR 150 I was using it for commuting to and from school and the worst terrain I went over was rough flat surfaces.
2. Having a seat that's to soft or wide isn't good for trail use because it restricts movement, it's good for commuter bikes but not for trails.
3. You can expect to attain higher speeds from a bike obtained from an LBS since there will be a lot less power loss do to better suspension, bike fit, and lighter weight.
4. Again the bike you get at your LBS will be better, do to reasons stated in previous answer.
5. Dept store full suspension bikes were never meant to be taken out on trails. Not sure what you mean by that last part though.