I've owned a number of pairs of Korkers. The first pair used their first generation soles (OmniTrax v1) and as Bamboozle noted, the soles eventually popped off. However, the rest of the boot lasted very well for me. They had an AquaStealth sticky rubber sole available and it is the only rubber sole that actually lived up to its billling of gripping. That prompted me to replace them with another pair of Korkers. I got a pair of fixed sole Korkers next, with the Boa lacing system and was extremely disappointed when the Boa dial broke the very first time I put them on. Their customer support was excellent and replaced the boots, although they no longer carried that particular fixed sole model, so they gave me the current year removable sole model, which had OmniTrax v3 soles. Something gave up the ghost on those boots too - the soles delaminated and one of the eyelets pulled out, and this pair was replaced by them with the current year model (Whitehorse). This model has held up well, although the right Boa dial out of the box was far tighter on the one wire than the other, and I'm just not a fan of the Boa lacing system. I recently got a pair of the Korkers Guide Boots (KBG) - they have removable soles and good old fashioned laces, and I like them took, although if you don't like a high ankle boot, you won't like them. I also picked up a pair of sandals at one point, which had OmniTrax V2 soles. They were an improvement over the first generation, but required a tool to properly fit the sole and I think Korkers finally got it right with their third generation sole. That being said, I find that I changed my sole out exactly zero times last year. So while the concept is interesting, I don't change the soles at all. They market it as somehow being better for stopping the spread of invasives, but I think it is actually worse, as there are more nooks and crannies for junk to get into.
So, to summarize - the current removable sole generation is good. If you need to change soles often, it is a nice option. I don't like the Boa lacing system at all - doesn't mean it is a bad idea, just that for me, it's created nothing but problems that I don't have with string laces. String laces just rip. I did have to have a number of pairs of boots replaced, which is a bit of a negative, but their customer service was great to deal with. I do like my pair of KGBs.
For sizing, I've found the most recent boots to be true to shoe size. For a size 11 shoe, a size 11 Korkers boot has enough room for waders and a thick pair of wool socks.
For the soles, the cheapest ones are $29 but if you want studs or the aluminum bars, expect to pay $39 through $59 list price.
As an aside, I recently picked up a pair of Patagonia Rock Grip Aluminum bar wading boots. They grip superbly and I actually have confidence in my steps again, with one exception. If you step on something that has an axis parallel to the aluminum bars (think a tree root on a trail, a small log in a stream, or a rock edge), you can take a lateral skid pretty quickly, especially if the item you step on is angled at all.