I've tried Esbits on a couple of trips; found that a GOOD windscreen is absolutely essential - otherwise, you can sit there watching water almost boil. Just never seemed to be a great choice to me.

I've also used a Pocket Rocket above freezing for about 10 years with no problems. As Lori points out, you can't put a canister and the stove inside a small kettle (specifically the Titan Kettle, which I really like) - that's a reasonably minor PITA, though, and the MSR website says there's a new and improved version coming out early next year that will fit. We'll see.

Below freezing, I revert back to the MSR Simmerlite, so I don't have to mess around keeping canisters warm.