It shouldn't really matter.

If you were on a p4, maybe you should turn it off but with the core i's you wouldn't notice a different between them on or off. (other than heat and power consumption) Although in some scenarios HT can benefit games that can use it.

I'd certainly leave it on if you are using a relatively modern OS like Windows Vista/7 or Linux. I'd be a little less eager to say "leave it on" with older OSes like Windows 2000/XP, but I'd still probably say it would be a better idea than not. The biggest reason why HyperThreading had adverse performance impact was that schedulers in older OSes stank and few Windows applications were multithreaded. Today, schedulers are a lot better and many applications are multithreaded, so HyperThreading is of some benefit.

My file server is a dual "Gallatin" Xeon unit (3.20 GHz, 2 MB L3, 533 MHz FSB) and it runs Gentoo Linux. I have HT on and it rarely adversely impacts performance as the OS not only knows what "CPUs" are real vs. virtual, it schedules threads intelligently as to load up the two real CPUs first, then go to the virtual ones. I can get as much as a 40% performance speedup by using HT in specific applications, but that probably has a lot to do with the fact the chip has the big L3 cache that all P4s except for the first three P4 Extreme Editions lack.