I used to use Esbit stoves years ago, but I was just never really happy with their performance. They're basically the same as putting your cup/pot/pan over a fire. You don't really get much adjustment to the flame.

After using them for a while I bought an MSR PocketRocket, which I was a lot happier with. It's just as convenient to use, but has a much more adjustable flame. Because of that, I find I use less fuel overall and get better performance when I'm cooking something beyond just needing to heat up some water.

The only downside to them is they start to struggle a bit once the temperature gets down to about 15*F or so, but if I warm it up in my coat pocket before hand it will work well enough to heat up a quick meal for myself.