Like a good Microsoft security citizen I installed BitLocker on my Infineon TPM-enabled laptop ages ago, well before we shipped the OS in late 2006. The nice thing is that I don't even know BitLocker is ‘doing its thing’ as there is no performance degradation that I can see.
 
But there is something you ought to be aware of.
 
If, like me, you use Sleep mode (aka Standby) on your laptop then it is possible that an attacker can logon to the laptop because Standby simply puts the machine into a very low power state (i.e. memory is still hot) and simply prompts me to re-enter my logon creds. Of course, if the attacker can break your password or smartcard PIN (and has access to the smartcard), then you have a whole swag of other problems to worry about! But humor me for a moment. So what I do, mainly when I’m traveling, in a hotel room or at conferences, is put the machine into Hibernate mode; the mode that writes RAM contents to disk and I have implemented BitLocker so the it requires me to enter a pre-boot PIN.
 
Now, when the laptop lid is opened, the attacker (or me) is prompted for the BitLocker PIN to load the image off disk. They could try to guess the PIN but the TPM is used to unlock the disk and has anti-hammer technology built into it. After every n failed attempts the TPM goes to sleep for x seconds - and x increases exponentially. The BitLocker recovery model works fine without the PIN should I forget it in my old age!
 
Sure, Hibernate is a little slower than Sleep, but it does have the advantage of requiring no power, and it could help protect your ass(ets) when you use BitLocker.