I've been using OpenRPG to run a game for the last few months.
It works fine. It is basically a specialised form of chatroom, with mapping tools and die rolling utilities. You can save the chat logs onto your PC, allowing you to review the game at a later point (or just have a laugh).
There is a whisper option, allowing you to talk to specific players without your message going to everyone logged in.
You have to change your play style a bit. Descriptions have to be shorter (basically, if you are typing for more than 30 seconds at a time, it is too much), which means that big speeches/ epic descriptions are out (unless you put them in handouts to be read offline).
You can upload maps into the whiteboard (which defaults to green), or draw onto it. Version 1.63 adds the ability to put text on the whiteboard.
If you have access to mini libraries (some are available from the OpenRPG site), you can add minis to represent characters.
There is a fog option (allowing you to put up a map and then obscure parts of it), but not all game servers support the fog option. I haven't used it, so I can't comment on how useful it is.
The servers fill up a bit in the evenings (US time). Because we play in the evening GMT, we tend to find that the servers just filling up as we are logging off. I've not had experience of it, but there are reports of lurkers logging into games and crashing them, so it is probably a good idea to password your game.
Frankly, I'd prefer to have my players round a table, because I don't get a sense of how the players are enjoying the game and you can't always tell when people are kidding (no visual cues). But my players are scattered across the country, so I don't have the chance to get them together. It is a good alternative though (especially against not playing at all).
The first time I ran a game, I had to fight the impulse to describe eveything in the old text game format (go SE, exits are W and NE, etc, etc), but you soon lose that.
If you set up the character sheets in the utility beforehand, it makes life a lot easier (the players can just hit the relevant skill and have OpenRPG roll for them).
I have heard other people grouse that the die-rollers in OpenRPG are biased to roll low. It's not something I've noticed as we've been playing, but then the people who have said that to me are ruleplayers, not roleplayers.
The best thing to do is to set up a game. Give it a go for a session or two and then see how you get on.
If you have any specific questions, or you want to sit in on a session (UK timezones though), let me know, here or by PM.
Good luck.
David