After reading this post, I explored the same issue, and I think a little clarification may be in order.
OpenLayers has a larger user base than Mapstraction, but still smaller than GoogleMaps (GM had more than 1000 posts in the last two weeks , the OpenLayers developer forum had 169 posts in October , mapstraction had 33. An absolutely fair comparison, because OpenLayers questions and Mapstraction seems to be a bit more complex types of developers and has less spam.I think both open layers and mapstraction support the use of google maps in Google TOS, wrapping full google maps js api.
One of Mapstraction's advantages over OpenLayers is that it has more direct access to googles api, while open layers support fewer google api ports, but it supports them more consistently on all supported maps. This list serves the issue, which is very good http://www.mail-archive.com/ dev@openlayers.org /msg02039.html
We are considering moving from GoogleMaps to mapstraction, and here are a few thoughts on how I started working with the Mapstraction api wrapper.
On their homepage, they link to a complex buggy demo that does not apply to the current version of google maps. If you delve around the git hub, there is a demo version of api v3 here https://github.com/mapstraction/mxn/blob/master/examples/googlev3.html
The code base on github seems well written, but their documentation is shaking. In fact, when you search for “mapstraction” on google , the Doc site link API points to outdated documents that also do not apply to v3. If you download the source of the git hub, there are auto-generated documents that close v3 but have nothing to do with a simple example. Their mailing list is hard to find, but I have not seen any examples either. It also seems that when they left code.google.com for the main source line, they still rely on it in all the examples to host js files and inject using the .js (apitype) setting. like this http://mapstraction.googlecode.com/svn/trunk/source/mxn.js?(openlayers ) I came across some chatter in a list service that suggested that google js often excluded necessary corrections in the code base. I did not find a good start-up document or email explaining their preferred scheme for linking code or injecting an api dependency.
These are all small issues, and they seem to have a good core team. So, if you need to support certain features of Google maps, they look like a good way to go.
For reference, here are their simpler examples https://github.com/mapstraction/mxn/tree/master/examples/