Well, this is interesting...
hyperthreading is actually not a great bedfellow for SQL - simply because SQL wants to do its own queing and threading, so what we see is the processors reckon that they can do a great job through hyperthreading and SQL server reckoning it is doing a better job, so we see heavy competition in the CPU. The giveaway is context switching and if Context Switches/sec higher than 5000 per physical processor then you have some real problems and should turn off hyperthreading.
However, not all CPU's are the exclusive use of SQL Server, and you can use the Affinity mask to help isolate / control which CPUS are available - leaving one for say the operating system and other apps... In that situation hyperthreading can help on the non-SQL processors.
There is a really good article :
http://technet.microsoft.com/en-au/magazine/cc137784.aspx
