1. It adds some fresh ideas and general improvements, certainly. I still think the game balance needs tweaking, though. (Cave Johnson in particular seems a lot less threatening now that his lemons don't slow affected players, and the low health speed boost doesn't help him as much as other bosses.)
2. The only bugs I noticed were the occasional wrong music playing and Saxton Hale occasionally failing to change colors upon using his rage.
3. Oh boy, here we go again...My short answer is that classes shouldn't be blocked, obviously (or else I wouldn't have gone to the trouble of reviving the combo in the first place). To elaborate...
First of all, let me just make one thing very clear: I do not want a mod war. Just recently, I had someone tell me that he felt obligated to stick around in my Rebalanced Class Saxton Hale server even though he wanted to leave. When I asked why, he said that he just didn't want Class SH to lose out to vanilla SH on the server list and give the vanilla SH players reason to gloat. I haven't tried to give anyone the idea that vanilla SH is evil or inferior or anything of the kind--apparently, they've developed this attitude themselves. Honestly, I hate this rivalry that has developed, and I don't want to encourage any further hostility--and adding a classes blocker most certainly would be quite hostile to the Classes fan base, I think.
With regard to gameplay, I just don't think that strictly limiting this mode to "Generic Survivors vs. Awesome Superboss" is the best thing for the mode itself. Without classes, there's no feeling of individuality or teamwork, and you have very little choice in which strategies you can employ (since your weapons are chosen randomly). As the boss, you're just faced with 10 clones of the same enemy--with varying degrees of skill, perhaps, but no major differences in capability. I don't think that's ideal. I believe that this is at least part of the reason that Class SH "caught on"--not just because of the appeal of playing as specific Robot Masters, but also because it helped to fill a gap that the classless version never bothered to address.
What I really think would be the best solution is to make a set of classes specifically tailored to Saxton Hale that people actually like more than other class mods, so that they don't miss the other classes. (If I recall correctly, Balrog himself included the ACS source for his classes blocker to leave open the possibility for someone to someday make a good set of classes to go with Saxton Hale.) To sum up my feelings generally, I'm hoping that this issue of combining mods will be resolved by improving the main Saxton Hale mod rather than by attacking other mods. Because if players are actually considering Class SH to be better than vanilla SH, as opposed to just an amusing novelty, I think that warrants some examination into how they consider it an improvement, rather than just writing such players off as having no taste.
4. Well, I already sent you my idea for an addition (a volunteer system for players who want to be the boss). I just hope something comes of it.