I think I'd probably prefer working with either ARM or MIPS assembly over x86, except for the lack of some of x86's built-in arithmetic instructions (like divide). As long as you've got an instruction set reference card handy (preferably in PDF format), it's not too difficult to work with any assembly language (I've done debugging in ARM and x86, took a class on MIPS, and wrote entire firmwares in 8-bit PIC assembly).

I was hunting through disassembled ARM code for performance bottlenecks and managed to speed things up quite a bit by manually removing divides that should've been caught by the optimizer from my C code. I didn't find it any more or less difficult than any other assembly language.