umm... I have the 5D Mkii... and have been shooting quite a few concerts. The limitation is the 12 min mark as mentioned.
But I use a "regular" Canon camcorder (HF10 in 1080p) for a straight "full-stage" continuous shot, wide angle... and use the Canon 5D mkii for
EXTREME close-ups during solos.
The Canon 5D mkii video is stunning... blows away most MULTI thousand dollar camcorders. The Canon 5D mkii has a 1-INCH sensor!
Go compare the specs to most semi-pro HD cams, they have 1/3 inch sensors.
Throw on a Canon 400mm L IS lens, and you get some fantastic video from the balcony.
If you have seen any photo examples in low-light from the 5D mkii, the video looks EXACTLY the same... except 24 frames per second
There are companies right now working on an external hard drive units (like the Focus Enhancement devices -- which use a FireWire connection, not HDMI) that will dump the Canon 5D mkii video signal via HDMI to a hard drive in QuickTime... can you say 8 hours of HD video storage?