Bit late.... but I have butterfly bars. They're.... OK. I've found they are aren't any better or worse than other options -but then again I think you tend to get used to any setup.
Pros:
-many positions
-usually very suitable for mtb drivechain conversions as mtb controls can easily be moved over -you can even put STIs/Ergos on as well. Pretty much anything goes with these things
-usually very wide compared to other options so they are very suitable for when you stand on the pedals and have to swing the bike side to side
-a lot of bar space so attaching extra things can be very easy
Cons:
-you'll need a new stem/quill as the position won't be the same as your existing bar, and possibly new cables. It took me a couple of quills to get the right one, so effectively I wasted the money on one of the quills.
-just me, but I tend to find I don't use the top parts as much -I find either the lower parts are too close for me to use if I can use the top parts, and if I can use the lower parts, the top parts are too far away. Sort of negates the many postions advantage, but that could just be me.
-they tend to be wide (again, that can be an advantage as well as a disadvantage).
I have butterfly bars on my converted old 1990's mtb secondary tourer/commuter bike. I used this bike to tour the Yorkshire Dales on and was pretty happy with the setup. All in all, I'm happy with them, particularly since I wanted a change from just straight bars and this came out to be the cheapest option (I used the same gripshift drivechain and brake levers, just had to buy new cables, bar tape and a couple of quills and I was good to go). However, if you want to spend as little money as possible, I'd say bar ends are the way to go, a very cheap addition.