I've got the Topeak MTX trunk bag. Ran it for a long while with the seat post mount. Did alright, but could never truly get it so tight that it wouldn't sway when I went around curves.
Ultimately went with another bike for commuting, where I made sure that it had eyelets for rack mounting.

If you do decide to go with the seat post mounting, keep in mind that at least Topeak (and I assume others as well) recommend NOT using it if you have a carbon seat post.

Agree also with the comments about SPDs. Really like the fact that I can walk around in them, without worrying about scratching up floors in the hotel.