I am using GoToAssist, and while it seems a little pricey, it pays for itself the first week of every month when I get the first call I can remote into instead of driving on-site. If not in the time and gas spent, in the agony of going on site for a problem that could have been handled remotely instead. Why leave your office if you don't have to?
I haven't had any problems with Windows XP, Vista or 7 or any of the server versions yet. When my clients can't get it installed, it is usually because their computer is too infected or broken to do anything, let alone install GTA.
My favorite part of GTA by far is the Unassisted Sessions. I use this feature for access to various servers, and for my regular client's computers I access frequently. The only other product I have tried was TeamViewer, but I wasn't impressed.
If you are running a tech business and you haven't pulled the trigger on remote a support service, seriously consider just doing it. Not only does it save me massive amounts of time, gas and wear/tear on my vehicle for many calls, but it gets me into machines I normally would not be able to service due to limited remote access options, and pure distance. I have even helped my business clients while they are out on vacation in far away lands. These are dollars someone else may take from you.
Furthermore, I can be connected to multiple systems at the same time, so I am potentially making 2-3-4 times the amount of money per hour.