I'm averaging about 8 flights a month at the moment.
Every airline can have a bad day, but - in general - Jet Blue are pretty good. I hadn't heard of them subbing flights to Virgin America, but it's be no problem if they did - Virgin America have the coolest planes in the sky and they're the only one I know of to have 3 pin regular 110v power sockets in coach.
If you are on a Jet Blue flight, you might want to to try and get one of their 'Roomor' seats near the front. You might need to pay a little extra for it, but the extra legroom is worth it. That said, it wont help width wise. Jet Blue fly A320's exclusively with a 3*3 seat layout. Most airlines reserve the exit rows for their frequent fliers so you might struggle to get one on Delta (a don't fly them often - only the occasional trip in to Atlanta). There's also a growing trend towards charging extra for aisle and windows seats and for checking bags (US Airways being the chief culprit). Don't know what Delta's policy is, but Jet Blue haven't gotten in to this yet.
Wooley.