Try as I might, I couldn't really get a bead on South Padre Island until I went there, and now it all seems so obvious. :)
At Isla Blanca, I personally know of some people who go back year after year, and there are probably tons of them. They often take the same site, and do improvements to it even though they don't own it--things like installing pavers on the site. One woman I talked to has a bunch of fencing and whatnot that she rolls up and takes with her at the end of the season.
It's very much a snowbird village, although there was one windsurfer who I know stayed there. We chose Andy Bowie because it was a lot closer to the North Flats sailing access, and I'm not sure we could have gotten a space at Isla Blanca anyway. As I said before, it does fill up during snowbird season.
There was some controversy last year because they raised the rates by kind of a lot (I can't remember the exact numbers) and didn't send any notice to people who had reservations, and who evidently had been paying more or less the same amount for years. They didn't find out about the increase until they showed up down there, and were not happy about it.
Traffic on the main street in SPI picked up noticeably between mid-December and mid-January. And you don't want to be anywhere near the place during Texas spring break (mid-March) because it's gridlock all over the island, as well as on the bridge between Port Isabel and the island.
I'm not all that crazy about how the Texas coast looks, and Isla Blanca is no exception. There's no grass--just some scrubby stuff with lots and lots and lots of stickers in it. (It's the same at Andy Bowie--whenever I'd take even a couple of steps in the "grass" behind my RV, I'd have to spend time pulling stickers out of the soles of my shoes.)
Basically, Isla Blanca seemed a little less ratty than Andy Bowie, but not enough better that I wanted to drive that extra distance every time one of us wanted to go sailing.
Let me know if you have any specific questions.