I've tried both IM and SubQ. With either technique, there's no need to use anything bigger than a 0.5" 29 gauge insulin needle.
IM can tend to aromatise more. Also left me feeling like I had a massive dead leg for days, although I'm told that this lessens the more you inject in a certain site.
SubQ is quick, easy and usually painless, although as megatron points out, occasionally it is possible to get a sore bump under the skin that lasts for a few days.
For me, SubQ is the winner.