Finally bought a back mounted one (smooth rotary), this summer and very pleased with it. Definitely prefer that setup to the ones placed on the front because it's more traditional to how the original arcade was with using the right hand for the spinner and left for buttons. It is surprisingly comfortable and natural to hold backwards like this more than a thought it would be too...
my only complaint, which most likely isn't the fault of the hardware, but how extremely fast the blaster moves, and since the game doesn't have the fastest framerate, there isa lot of frame skipping if you move it too fast which makes things much less accurate.. compared to the smooth as silk original arcade controls.. but if you can get used to having to move it very precisely it's definitely a preferred method. I played a couple games the other night back to back, one with rotary, one with regular d-pad, and both received similar scores, around 650k, but the way I actually play and move between the two are vastly different..for instance with the d-pad, I have to constantly keep moving and rocking back and forth, for getting power ups, I normally "ram" into them instead of waiting for them to come to the particular part of the web they come in on, where as with the rotary it's too twitchy to do that, so I make sure to zoom over to the spot and sort of "lock" it in before it reaches the rim, otherwise you'll likely miss it if you keep moving... so yeah, things like that make it play much differently for me anyway.
It is kind of annoying having to switch over to player two controller to play the first and third sets of bonus rounds.. for some reason it doesn't allow you to play the green line bonus level with the 2nd controller though. I found it much easier to complete with the d-pad for that level. Don't even bother trying to play the bacon bonus levels with the rotary though.. it's impossible.
But yeah.. i can't believe I waited so long to get one of these, especially considering it's one of my favorite games of all time, and been playing since the 90's.