Nothing wrong with dryhopping in the primary, but a few things to consider:
1) You don't want to dryhop for more than 7-10 days. The hops start to add a bit of a grassy taste to the beer after that.
2) In my experience, a dryhopped beer has the nice, fresh dryhopped aroma for about 10-14 days before it starts to dissipate pretty rapidly. For that reason, I typically dryhop cold in the keg starting whenever I start to serve the beer instead of in the primary, as that dryhop aroma will fall off pretty rapidly. The downside of this method, of course, is I typically have a bit of hop dust in my beers for the first 4-5 days, but nothing that bothers me. One of these days I'm going to invest in a Randal/Hopback to solve that slight issue.