The old stigma used to be that ASP tended to be for larger companies and PHP for smaller business, enthusiasts and freelancers.
This mostly used to be due to the pricing of servers, with windows servers being more expensive.
I started the transition from PHP to ASP about 6 months ago, after I found that anything based in the larger companies wanted this scripting.
I've very much enjoyed learning it so far but am far from an expert, and I agree with Pho on many of his points.
Its another string to your bow if you can find the time to learn it, it does seem to be becoming more and more popular (from my limited experience at least)