I've been using Mapstraction fairly intensively on a couple of projects.
On the plus side it's a very well written abstraction layer and works tremendously well. The code is well structured, logical and handles everything you'd need for basic mapping. With a bit of effort it's easily understandable and can be extended. Also the underlying bespoke map objects are still accessible, and you can write specific code against them.
Against that there are a few issues. Of course because it is an abstraction layer if you want to do anything specific to features unique to say google maps, then there is additional effort required over a straight implementation. Bing maps is also only currently implemented on v6 instead of the improved v7.
Personally I find the swap between Google, Bing and OpenSourceMap a real positive, especially in view of licencing issues and prices with Google and to a lesser extent Bing.
So if you need a lot of google-specific features and can handle the licence issues then go with google, otherwise I'd recommend mapstraction.