We use a lot of Microsoft technologies at work, so I was curious to see how well ActiveRecord would work with SQL Server 2005 (I tested against the CTP edition).
Why not stick with MySQL? Well, though I use MySQL personally for some applications, SQL Server 2005 is significantly more sophisticated. If it works well with AR, then I get the best of both worlds...
The feedback is mixed. On the plus side, getting up and running was painless. You won't be able to do windows integrated auth but SQL Server auth works fine and once the user is created, you're up and running.
On the downside, the SQL Server adapter is broken. I logged a bug in the Rails tracker (see link for more details). In some cases, the adapter can confuse data in the query for its instructions, and end up running the mangling the query. We either need a much more sophisticated set of regexs or (preferrably) a way for an adapter to get the information it needs without resorting to parsing the query.
Interestingly the MySQL adapter doesn't need to resort to regexps at all, nor do most (all?) of the other adapters. For the moment, I'm sticking with Rails on MySQL!