If you are opertaing in a high-security environment, then you will want to use SMS Remote Control rather than the other tools available. One reason is that SMS 2.0 has several layers of security for remote control.
The first layer (which most tools have) is that it prompts the user before the session begins. This allows the user to close any sensitive documents opened (but it still allows the Remote Controller to re-open them!)
The second layer (which most other tools do not provide) is NT group membership-based security to access the remote control feature in the first place. This is accomplished (this only applies to secure environments) through a "Permitted Viewers" list that you add any users that are allowed to Remote Control computers on your network.
Another reason to use SMS is that SMS 2.0 logs the usage of the remote control session for you to audit or investigate. Other tools are pure point-to-point solutions that aren't technically secure. If you can purely rely on your users descretion for security (in other words, if you have an in-secure environment), then one of the other tools is sufficient. I think that WinVNC is even freeware, right?
Another thing about the SMS remote tool....if you have a large helpdesk that will use the tool, then the SMS admin console has serious limitations. Mainly a steep learning curve and the lack of easy searching capability. There is an easy way to get around this, though, because SMS created a way to run the Remote Tools feature without using the SMS MMC (although the MMC must be installed on the workstation). If you have a site up and running and the admin console installed, simply run REMOTE.EXE and check it out.