A macro flash was very high on my wishlist, finally decided to buy the Canon MR-14ex because of the good reviews, and I didn't want to buy a Canon after I bought a cheap fake substitute and were very disappointed about the result of the cheapycheapy one.
I mounted my EF-S 60mm on my 400d, clicked on the macro ring, switched on the unit, and the first shot of some coins where excellent.
In low light the focusing lights are usefull, but they switch of very fast. Unfortunately Canon knows this problem, because the manual mentioned that this function can be changed in personalised settings ( custom functions ) on the MR-24ex by double clicking the shutter button ( switches the focussing lamp on or off.
The exposure measuring trough the lens works fine. I am now trying to set the flash ratio of one of the lamps higher so a more natural look is given with some shades on one side. My 430ex can help with that because of the fine working Master / Slave function of the units.
Flash exposure bracketing is also possible. For the real freaks, HDR macro photo's can be made combining the three different exposed pictures in one. You definately need a tripod then.
It is expensive for a non-everyday item, but I think it's worth all the money. the unit comes in a nice handy polstered leather (like??) bag, and needs 4 AA batteries, you can find those every-where.