It's anyone's guess how successful W8 will be in the long run. Windows market share on the PC is still nearly 90%. The big question is, how will it do in the tablet market?
If you look at the WP7 case, it's not promising. Even after nearly two years, WP7 doesn't even have 5% market share. Sales aren't sufficient to support most developers.
That said, it's an earlier phase in the development of the tablet market. It's quite possible that MS will fare better here. One reason WP7 didn't do so well is that all of the other hardware manufacturers (excluding Apple, obviously) had already jumped on the Android bandwagon and were doing well there. The same can't be said of the tablet space at this point.
Every non Apple hardware manufacturer wants to have a real horse in the tablet game. It could be argued that the Android tablets were an opportunity for manufacturers to get their game in order for W8 (this, and mobile experience, for those that develop phones). There will be a ton of marketing money behind W8 in the coming year, driven, in part, by all of the hardware partners that MS works with.
MS has a great development tool chain. They also have excellent UX research backing W8 and the experience is likely to be much better on W8 for the average tablet user.
My guess is that by the time W9 ships, MS will have significant market share and they will have sorted out most of the kinks with W8. It's early days right now and there are likely to be some bumps. This will be a bit like Vista vs. W7. The first version was pretty good for what it was and they nailed it in the second release. The difference is that this time, the "first version" is quite good, given the significant departure from the past.
If you've got Windows experience, I would say, go for it. In particular, if you've done any Silverlight or WPF, the learning curve is mostly behind you. If not, you've got several avenues to approach it: HTML/CSS/JS or C#/XAML, or even C++ with XAML or DirectX. There will be a learning curve, of course.
If you're looking for a new opportunity, to be honest, I can't think of anything else on the near horizon that compares. It could be, meh, but it could also be huge. Good luck.