depends on what you are doing and wanting.
1. Security.
-I've heard tell that SP2 is a little more secure, but that also doesn't cover computer stupidity and the point and click lifestyle of not reading or knowing what you click on. No security update in the world is better than a good Antivirus/Antimaleware program.
2. Reliability.
- The Service Packs add together all of the reliability updates, so this might be a plus.
3. Program Acceptance.
- Some games need SP3 to function, although I think this is more of a gimmick to get people on board with the SPs than actual necessity.
4. Support.
- Microsoft no longer supports SP2. This however is a very bad reason to upgrade.
Basically, If SP2 is doing everything you need it to, then don't mess with success. If you run across a program that you want/need and it needs SP3, then upgrade. If you have a reliability issue, or multiple issues, then upgrade as you need SP3 to get other updates.