Having run with and without FV2, I can say on my MBA laptops (2 of them), there has been no performance difference for me (coder/I.T. Manager). With all the information that can be put in temporary/cache files on your main drive, in my opinion FV2 is essential.
If you want to use TrueCrypt on external drives, no issue. But your main drive which applications can and do store data without notifying you should also be encrypted completely - which is where FV2 comes in.
One note you should be aware of is that having a TB or FireWire port makes any memory based encryption key vulnerable:
http://www.frameloss.org/2011/09/18/firewire-attacks-against-mac-os-lion-filevault-2-encryption/
The article mentions ways to avoid those problems as well.
I have lots of information I don't want people to access on my laptop - family photos, copies of all my taxes for the last 5 years, bank information, photos of my passport / ID documents, and more.
Two weekends ago I had my backpack and gym bag stolen out of my car. In my backpack were two MBAs (personal / work) and in my gym bag was my iPad. I remote wiped my iPad using Find my Device in iCloud (thankfully it was on ATT's network) but my two laptops were encrypted and the thief wouldn't have been able to use them. I wasn't the least bit worried about identity theft or losing money due to the laptops being stolen. That peace of mind was well worth the small amount of time it took to enable FV2.
(As an aside, I did get all my equipment back - $5000 worth of electronics and work stuff)
