There's two major things I'd consider before doing a major operating system upgrade of any kind:
1) Does my computer support it, and
2) Do I need to upgrade?
The first can be answered by looking at Apple's system requirements page: http://www.apple.com/macosx/techspecs/
Most computers within the last 3 or 4 years should support it, if I remember correctly.
The second is a more personal question. If you're happy with Tiger, there may not be any reason to upgrade. That saying, there are a lot of new things with Leopard. A little bit of a new look, I think it's cleaner overall, and the Time Machine features are very handy. Just make sure you have something to back up to: Apple sells Time Capsules for over a network backups, but something a simple as an external Firewire or USB hard drive will suffice just fine. Once again, specific requirements are on the same page as above.
I've had no qualms about going to Leopard myself, and I think it's a fine upgrade. However, Apple will probably be releasing the next version of Mac OS X, called Snow Leopard, around June/July of this year. My view is that if you've waited long enough with Tiger, it might be worth it to just stick it out until Snow Leopard. It might require more hardware though, so that's something to be wary of. An Intel Mac should do fine.
Okay, that's a lot of stuff... Conclusion:
1) If you have a relatively recent PowerPC Mac, Leopard is probably a good upgrade. I have no problems recommending it.
2) If you have an Intel Mac, Leopard is a good upgrade, but it might be worth waiting until Snow Leopard. This might also apply for some PowerPC Macs too, but I can't find any system requirements online.
Right now I've got an iMac 10.4.11. I've been thinking about upgrading it to version 10.5 for various reasons but I need to know if it is worth it. What are the pros and cons of upgrading? Are there any reasons I should stick to what I've got or should I just go for Leopard?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic