I've experimented with OpenRPG. I've just finished playing around with Fantasy Grounds II, and you can see the first stage of my ruleset development on that other blog, which I'm abandoning for being a pain in the ass to update.
Speaking of rectal pains - I've been completely disillusioned by Fantasy Grounds. I poked and prodded, and it had it's flaws (unintuitive in many ways, painful load times, and a kludgy way of handling multiple rule systems, as well as requiring actual development on the part of DMs wanting to put their rulesets into the system), but it had some *stunning* aspects which made up for it (the dice rolling made me cry at it's beauty, it's ease of spontaneous graphical resource distribution, and a few more bits)... and you can feel the massive "but" coming on, I'm sure.
I'm South African. Bandwidth is expensive for me. But even if that wasn't a problem - the first time someone joins a campaign, THE ENTIRE RULESET YOU'RE USING (approximately 12Mb for the default, d20 ruleset) is archived (down to about 9Mb) and transferred to the players - whether they have used the ruleset before, have purchased or developed it using the full client, under any and all circumstances.
I'd forgive moving this stuff if it was small - moving a ruleset made up of XML docs compressed to a few hundred Kb isn't too bad. And I'd forgive the massive transfer if it was an absolute one-time thing - if the first Call of Cthulhu ( <3 ) download resulted in never having to download that part of the CoC ruleset ever again.
As it is, Fantasy Grounds (and their developers, SmiteWorks) have lost a possible customer (despite the awesomeness of their name almost redeeming them...), a possible for-free content developer, and an outspoken fan.
In other and completely unrelated news: I'm developing an alternative to OpenRPG/Fantasy Grounds. As sad as this makes me, development will be for the Windows platform, and I'll be working with an old friend of mine, who goes by the ominous handle of "Anarchist".