I've used Zoomers for a couple of years to improve my flutter kick during kicking drills. I try not to use them during regular swimming sets as I find they throw off my timing. They've helped me build up my kick especially in backstroke.
I just got a monofin (last Thursday) to improve my butterfly kick; so far I've not seen any improvement, but my abs are sure sore! I figure it will take a few months swimming regular drills with the monofin before I notice results.
If your goal is general swimming leg work then I would recommend the TYR Split-Fin, if you are looking to improve core strength and your dolphin kicks then I would also recommend the Finis Shooter Mono Fin.
Paul