Points.com does have its flaws for sure. But with GPX trades you can get very good deals, it all depends on what trade you agree to. For example, you can set a specific trade for a determined price. Then you just wait to see if anyone accepts your offer. If no one accepts it you pay nothing. I've been surprised what trades some people will accept. Here are some trades I've made on points.com (GPX) over the last few months. I think they were very good deals, all things considered. I usually transfer all of my Continental miles to Amtrak once I have the required increments of 5,000.
I traded 2,000 Delta miles for 5,000 Continental miles for a price of $20.
Traded 1,000 Delta for 3,000 Continental for $10.
1 Airtran credit for 3,000 Continental miles for $29.
1,000 Priority Club points for 1,000 Continental miles for $5.
My really large trade was more expensive. Last fall I had 23,000 Delta miles I wasn't going to use. I was able to trade with someone for 23,000 Continental miles. It cost me $260, but that bought me a two-zone roomette award for me and my sister. That was more than worth the $260. It was worth the original Delta miles, which weren't even enough to get a free flight on Delta. It was also cheaper than buying points from Amtrak.
Anyway, I would definitely play around with the GPX trades. Also get on their emailing list because sometimes they've emailed me specials. The thing that is a rip off is the "point conversion." With that you always face great devaluation.
Good luck!