Linq, ick. It's a performance nightmare. No one knows what it is actually doing in the background so firstly, everyone writes horrible O-cubed performance with it, and then it is a series of random poking around to try fixing it.
Depends if you're using it with ORMs or not. It's equally as useful with data in memory. Definitely slower for some things though (yes, it's always slower in one sense... But its also easier to make parallel, so it can even out in the end)
