It installs a whole host of security updates, which is a good thing
As a result, it could run a little bit slower
Most people would consider it to be like installing Norton Anti-Virus or another similarly chunky anti-virus program - you know that it will make your PC run a little bit slower but you still want it.
As with any major update, there is the risk that it will not install properly and cause your PC to have problems, possibly not even starting back up properly. The best advice here is to make sure that you have a reliable up-to-date backup of anything that you can't afford to lose separate from the PC (e.g. on CDROM) and test it before downloading the service pack.
