'09 Diamondback Insight 1, '05 Trek 3700 (currently under reconstruction into a street/cross trail bike), (Vintage model) Kent Supreme 10 speed road cruiser, BMX (just for fun), Trek Multitrack 720 and a homebuilt recumbent low-rider.
I ride a Diamondback Insight 1, and have put over 3000 miles on it. I bought mine slightly used from a LBS, and got a good deal on it. Three years on from its "birth" it's still running just as well as ever. I regularly ride mine for 30 + mile trips, and I still love it.
They're light, comfortable, and quick. The components seems to hold up well, and I've had no real problems except popping a spoke recently after hitting a pothole hard on a paved trail.
The guy at the local bike shop that fitted a new spoke and true'd the wheel, said that the wheel and hub bearings were still in great condition, so I was pleased with that news.
They're pretty good from stock, and the only real modifications I've made to mine are swapping out the seat to one made by Giant (for a little better fit), and adding ergo hand grips (for comfort) and bar ends to give me a change of hand positions on longer rides. I've swapped out the pedals to to BMX style studded pedals to give me more grip, because those stock (plastic) pedals arent great when wet. You'd probably want to do that with any bike you get anyway... we all mod them a little to suit our own requirememnts, right?
I have a Trek 3600, and an older Kent 10 speed road bike (now converted to a cruiser) too, but the Inisght is really my favourite for paved riding.
I've never ridden the other model you mention, but I'd have no problem with recomending the Insight.
I am not familiar with the Gravity bicycle that you mention. However, I did buy a used Insight I this July and have ridden about 500 miles mostly on paved trails and some roads. I have added a rack and front bag and changed out the stock pedals for clipless. I was a bit nervous as some LBS people look down upon Diamondback but I've quite enjoyed it. I took it to the local Trek shop for a tune-up and the staff there didn't adjust the gears well so there is some clicking in high and low gears. But the bicycle itself has been a good purchase.
I've had my Diamondback Insight 2 for a year now and have approx. 2,500 miles on mine. I've replaced the seat for one that fits my sit bones better, and I have replaced the tires with Continental City Rides after wearing out the original tires that came with the bike which lasted about 1200 miles. Better tires make a world of difference.
Other than that I've had no issues with the Insight.
It's been a good dependable bike and I do all my own mechanical work, and once it's adjusted correctly, there won't be anymore issues than you will find on any of the more popular brands in it's class.
Most bike shops don't carry the Diamondback brand, and that I feel is the rub with the bike shops as why they may look down on them, but if you know how to make your own adjustments, it's as good as many of the LBS brands in my opinion. I've got three other bikes that are Treks and I ride the Diamondback more than the others.