Sorry about the ROB boards not coming, we've more or less died since OS switched to MK.
One thing I've noticed is a lot of talk about juggling and hitting the blindspots; a good ROB is able to avoid that. The key word here is good; a bad ROB that relies only on spotdodge-dsmash and the like will be ***** in this matchup. ROB has to rely a lot on his spacing moves to win. ROB also has a better general ability to manipulate the match (grounded gyros, camping, etc.). However, his spacing and control are extremely skill-intensive, meaning that even a mid-level ROB vs. Sonic matchup is extremely different from the top-end group, and is probably about even.
ROB also isn't THAT weak on his blindspots. His ground blindspot is closer to 60°, not 45°, which is a bit harder to approach (and he can do retreating short-hopped fairs in order to keep you from taking advantage of it).
In the air he is weak from behind and below, but luckily Sonic isn't that fast, so it's no problem. O wait, Sonic is fast, you say? .... ok, yeah, he is. Sonic can actually take advantage of the aerial blindspots, so ROB should rarely float down onto the stage. ROB can do a decent job of minimizing Sonic's access to the blindspots with wise use of bairs and suchlike. If the ROB doesn't actively minimize your access to the blindspots, he's in trouble. If he does, then it's pretty straightforward for him.
O, and if Bair is spaced and timed properly it's not punishable by most characters. I'm pretty sure Sonic's included there, though I don't know for 100% certain. Because of Bair's weird timing, however, that's a lot easier said than done.
Neither character should count on gimps. If one character would be able to get them I think it'd be ROB, but that may just be my bias. In any event, they'll almost be irrelevant; most of the kills are going to come from kill moves, and ROB's are slightly better than Sonic's (though not much).
In the end, 55/45 to 60/40 in ROB's favor sounds about right to me. I think ROB improves more with skill and matchup experience; his spacing and timing is pretty precise on everything except dsmash, really.