I've been using this for about a month now, they have carried on where they left off with 2011 in that the impact on system resources is minimal.
Doing a full scan of C:\ (300GB Partition 80GB used) uses about 20% CPU time* and took about 10 minutes.
This is now my number 1 AV without doubt! Well done Symantec

*System: Intel C2Q Q9400, Asus P5Q Deluxe P45, 8GB DDR2, 2x WD 1TB Black RAID0, 8800GTS, Windows 7 Ultimate x64 SP1