I've been using Mapstraction to implement draggy maps in our platforms at work for the past month or so and thought I'd share some of the problems I came across so, should you decide to do the same, you know what to watch out for. The issues I've come across below I've fixed for the APIs with which I was working and I'll be submitting the changes back to the project in the near future.
I should say first that Mapstraction is an awesome piece of work that frees you from having to commit to one mapping provider. When clients don't want to pay for maps so you give them Google and then change their mind when Google start inserting advertisements, all you have to do is change one line of code.
1. Patchy implementations
Mapstraction abstracts the lowest common denominator set of functionality for its supported APIs. I've been working primarily with Multimap and have discovered that, for this API at least, some API classes are missing methods or have non-working implementations.
In the Multimap case this is just a few methods here and there but the thing I found annoying was that, rather than the method just being empty or throwing a "not implemented" exception, the method was populated with the code for one of the other APIs.
2. Incomplete events
The only events catered for by Mapstraction are "move end" and "click" which is severely limiting considering the vast array of events available through the native APIs. Events like "zoom", "open bubble", "change map type" etc aren't there.
Arguably, abstracting events is probably the most difficult part of the process as you have to take into account differences in when similar events fire, orders that they fire and event arguments that are passed back to the handler.
3. No namespacing or prefixing
4. No standards enforced on the source code
There appear to be no coding standards enforced on the source. It's full of undelared variables, dodgy constructs and some bits that just aren't formatted well, making the code difficult to follow.
The likes of Yahoo! run all their code through JSLint before it goes anywhere near deployment and with a library of this type I think at least a loose Linting is a must.
To sum up
If your integration is complex and requires a lot of event handling and dynamic manipulation of the map artifacts then Mapstraction isn't quite there yet but for most other things it's worth filling in any holes yourself for that freedom of API.