Having never used a CMS system before, I'm just starting evaluating .net CMS systems, with the two most attractive options seemed to be Umbraco or Orchard.
Re Umbraco, I'd prefer to use the MVC flavoured v5, rather than spending lots of time learning XSLT for version 4.7, when that has come to the end of its development cycle. But, is now (May 2012, very soon after the release of 5.1) is actually a good time to start out on the Umbraco learning curve, as there doesn't seem to be the depth of support and documentation out there for v5 compared to v4? I'm sure it'll be improving all the time, but as it'll be my first CMS system, it seems a little scary leaping into a brand new, rapidly developing system.
Sorry for the subjectiveness / vagueness of the question, but any useful comments comparing the probable learning curves of Orchard and Umbraco 5.1 would be very much appreciated.
I agree with the above poster: You don't need to xslt with umbraco at all, v4.7 supports Razor.
Its a good time to start learning v5, but I would not promise any deadlines to clients any time soon. There are a lot of performance related issues that make it unsuitable for all but the smallest of sites right now - If your deadline is 6 months out, you could probably bank on V5 being stable by then, anything less than that, and you are better of sticking with 4.7, which is really fast, full-featured and a pleasure to work with.
Depending on the site, and if you stick with razor, the upgrade might not be that hard if you ever need to do it. I just spent about 3 months developing a v5 site, and because of the problems had to convert back to 4.7. The conversion wasn't that hard, and now being back in 4.7 makes really appreciate how stable it is.