I've bought from Model 1 Sales twice, and I've been mostly pleased.
First, I had a gunsmith friend build a 16-inch carbine for me based on a Model 1 CAR kit, which I went with based mostly on price since then it was already a financial stretch for me to get into an AR15 at all. With the exception of the handguard, the upper unit (barrel, upper receiver, sights, BCG, etc.) has been excellent. I thought the handguard was too small and flimsy, and it did little to dissipate heat. I replaced it with a Bushmaster M4 handguard really quick. Some of the lower parts, however, have not held up well, which to me gives credence to the complaint that Model 1 uses cheap, out-of-spec LPKs. I've had to replace the bolt catch and trigger pin, and I upgraded to an RRA 2-stage trigger because the original was pretty bad. I've inspected the other LPK components, and everything else appears and functions fine. Ultimately, I have really loved this rifle, but I did have to work some bugs out of it.
My second experience only involved the purchase of an assembled upper, sans the BCG. I went with Model 1 again on this occasion because I wanted a government profile 20-inch barrel, and Model 1 was then one of the few sources for an affordable upper in this configuration. And, since this upper was intended for another assembled lower that I already had (not the same as my 16-incher, BTW), I had no worries about Model 1 LPK components. Again, I was not happy with the handguard, and I replaced it with a more sturdy one from RRA. Otherwise, the upper has been flawless and very accurate to boot.
In my opinion, Model 1 makes great assembled uppers, only with subpar plastic handguards. Their LPK components are also spotty, so I'd avoid them in favor of a more reputable maker. If I'm in the market for another AR15 upper, I'll definitely consider Model 1's stuff. Even with their flaws (and spending money to correct them), Model 1's price point is hard to beat.