I use msstyles but not Style XP. I don't need to use a "product" to patch uxtheme.dll.
There are a handful of good msstyles floating around but I the other day while bored I did a survey of the last 100 skins submitted.
3/4ths of them are Luna with a differnet color (that is 75 of 100). You don't see that on WindowBlinds because users can recolor their own skins dynamically.
Only EIGHT were original creations, the rest were OS ports (mostly Aqua with buttons on the wrong side).
While I'm sure over time there will be better ones out there, the real question is, can msstyle attract high quality expert skinners (ala Treetog or Dangeruss or Alexandrie?). Would people of that skill be satisified with the limitations of that format? And nearly every top skin author I've known is older (i.e. over 20). And that's very bad news for anything whose claim to fame is that it doesn't cost a whopping $20 to get all the benefit out of ($20 is a lot if your only source of income is your allowance and you have to borrow your parent's credit card but to most adults, $20 IS effectively free).
But quantity wise, yea, msstyles is doing well. I'm sure in a year we'll see things like "Well there's 2500 msstyles and only 2000 WindowBlinds skins!" Even though 2000 of those skins are just Luna recolored.