If you love, live and breathe critical care, do it (in a good internal medicine/multidisciplinary program). If you don't obsess about it, don't. It's a very easy, straightforward, decision if you're passionate. Most GOOD critical care docs are. (One needs a lot of passion to keep up with the amount of literature and nights/weekends.)
If you're unsure, then go practice as an attending, and see if you miss critical care. Most unsure people don't.
To answer the other part of your question: best year of my GME. Royal waste of money for me, but the step up in medical thinking and echo skills was palpable. I am an anesthesiologist, so it may be a different experience for EM docs.
To me, it's the best part of my work. Nothing makes me happier than changing a disease course, possibly saving a life. Futterwacken, baby!