Size matters, what size you need is important.

In my experience, if the fork on the bike is really nice, you may see some improvement, but not much. For example, on several Ironman bikes, I've gone back to the OEM fork.

However, on an Ironman I just finished, I went with a Kestrel EMS Pro 1" threaded fork and it's fine, rides smooth. I can tell it's lighter, geometry seems a wee bit quicker.

Also, on a Klein, I swapped out the lugged chrome Tange fork and went with the Performance / Nashbar 1" threaded fork, and it rides nice AND has clearance for 700x28's.