In case people with first-hand knowledge don't reply, I offer my own conclusions from reading a number of threads at various forum sites, all anecdotal.
ARES: People on TheOtherSite seemed all excited about it because it is affordable and a non-permanent drop-in, but a number of reported failures quickly deflated that. Failures included the operaeting rod popping out of the piston, breakage where it screws into the gas block, etc. They also reported a lot of trouble getting the company to respond to their emails and calls. But keep in mind that happy customers rarely post on websites.
POF: While I've not read first-hand accounds of POF failures, I read hearsay posts about them having stoppages during classes, and I got the idea some were caused by weakness in the operating rod (IIRC). Personally, I think their rails are big and ugly.
LWRC: I've have yet to read a negative report on an LWRC. Their design seems stronger as the operating rod is supported over more of its length. In general, it seems like they are trying to build the best piston rifle possible rather than focusing on cost. They seem to be focused on getting military interest, which requires robustness even in sustained full auto. But they are still pro-consumer (totally unlike HK). I even understand that Larry Vickers (of HK 416 fame) owns an LWRC and recommends it for folks who can't buy an HK 416 (i.e., all of us). Finally, their latest bolt carriers are TeflonNickel coated.
The Bushmaster piston is a POF but missing some of the nice features like the coatings.
I've never heard of the Kurt Kustom conversion. Sorry.
So based on all the above, I decided to buy the LWRC. Don't ask me how I like it because it's not arrived yet. But my deciding factors were based on reliability, ease of maintenance, and customer service. Cost really wasn't a factor in my decision, but I don't think they cost any more than the POF or a high-quality DI AR (Colt, Sabre, etc.) plus an ARES. In fact, I think they're less.
Again, all the above is just based on my poking around various forums including TheOtherSite, this site, and the LWRC forums. You'd be a fool to take any of this as gospel, and should perform your own research, etc., etc., etc.
ETA: LWRC does conversions as well as selling uppers and complete rifles both directly and through dealers.