A little basic...
The board was sparse and half the game window's space is taken up by Sonic and Tails just standing there. The bonus boards and single-slot slot machine are nice touches, but don't do much to justify how open and empty the layout is. The collision is largely passable, but the flippers don't seem to hold any power to them, and there's a visible delay between when they flip up and the ball gets going with barely any speed. With the bumpers helping out it's not as obvious, but in the bonus rooms it's a glaring fault that makes getting the ball up past the bottom third of the board feel like dumb luck simply because unless it hits the top of one of the lower bumpers it's not going to get any higher.
simply put, what you have here is a foundation, and while I've seen you saying Flash couldn't handle much more, I've been having terrible lagging issues due to some Firefox bug or other that's been going on and it's about the first thing that's run decently for me on this site in a week. I don't know how you're handling your collision logic, but there shouldn't be enough here to slow things down by even a remote stretch, even for Flash. :/
And, on a more minor offense, while I had no trouble finding the music and ball options, the fact I couldn't hear the chosen audio until I started the board authentically irked me. Your menu is bizarrely silent, and no harm could come from it playing a little something something.
The physics are decent save the flipper collision, and this doesn't feel like a lost cause, just very aimless. Give the board more, or make the space it consumes less, maybe add some rollovers or an orbit shot, something. If you're already having issues keeping the game running smoothly a ramp or two is probably out of the question as then you have to handle layering, but I find it hard to believe more couldn't get into this. :/