I'm not sure if there is a thread that discusses this, so I'll just discuss some observations I have. I was previously playing at the $3.50 level until I could be bankrolled for $7.00.

First thing I noticed is a smaller player pool. On Sunday evening, Pacific Time, with 80k players on Stars, and a 45 man couldn't even fill up. The games fill up a bit slower compared to $3.5. With the smaller pool, I wonder if I will notice that more games will include winning regs.

I've only been at $7 for a few days, so it will be interesting to see what my results will be like going forward. So far, I had profitable stats at $3.5, and I believe I'll be a winner at $7.

Do you guys notice a big difference between these two levels? What about when I move up to $15 down the road? Will that be a noticably smaller pool of players; or any other differences in skill or playing style?

I can't see much difference. If you have a sharkscope subscription, you can select some high volume regs and see how they are going on both and compare their ROI on the $3.5 and $7. I recommend getting the names from the sharkscope leaderboard $2-5/4-6 tables.