Now, of course many levels suck. I just got back to it out of boredom, and found out that many levels are literally ring/monitor rush, with few or no enemies at all (so I really doubt Sega ever used this thing to get ideas for Sonic 4). So, the idea of this topic is to post levels that are good (or at least don't suck so badly).
Aztec Marble zone 1
Dunno if it's good, but at least it's quite playable (and with a decent length). It even has a pretty huge U half-loop, and you must use momentum here to get through it. Sadly, the engine fucks up and thinks that walls aren't walls >_> (hint: don't press anything but down and you'll be fine). By the way, aren't there any yellow springs in this engine? There are a couple of red springs that could be yellow =/ But besides that, the level is generally good.
PS: yes, it's the UK site the one linked. For some reason whenever I try to get on PlaySega, I end up in the UK site instead of the US one. Oh well (I'm from neither region anyways, so I believe it's just guessing).