I've got a rifle built using the JP Enterprises Grade 1 upper, had it a little over 5 years now.
I remember fiddling with the gas block a bit when I first got it then finally I just left it be, I set it to be reliable with the softest shooting low pressure round I might use then pretty much left it at that. There's a little bit of increased recoil if I use hotter rounds but that's about it.
The gas block is first rate as far as machine work and finish go.
The handguard, it's aluminum and SOAKS up heat from both the sun as well as rapid fire. To deal with the fact that it conducts heat so well I went and wrapped my handguards in 550 paracord. Definitely improved the general uncomfort of a metal handguard(got real hot in summer sun and real cold during winter). Plus I managed to much improve the overall shape and feel of the tube, area where the twist pattern is raised forward on the handgrip is actually a pinch point for thumb on one side and index finger on the other side to promote consistent grip. The custom knife grip inspired 1/2 twists fall nicely under the joints of the fingers and are an improvement in my opinion.
Here's the link to pics of the rifle....
[url]http://home.bak.rr.com/varmintcong/jpar/jpar.html[/url]
I think JP Enterprises gets a fair amount of parts from DPMS. DPMS is like the only supplier I know of for lefty uppers and bolts, I wouldn't be surprised one bit and if you asked John he'd probably admit as much. Order off their mail catalog, you'll probably find a LOT of stuff in it that isn't on the website.
I know when I ordered my upper that I was a little let down when I learned there was more to it than was offered on the webpage. At the time I ordered my rifle they had this really kick ass and wicked looking SOF and SOF2 upper that also came with a harmonic dampener to help tune the rifle to a load(like the BOSS II setup on Weatherby and Brownings).