We were an early customer of the API and were using it in ways it wasn't designed for so we encountered some problems early on ... the team over there consistently under-promised[1] and way-over-delivered. They were purchased by a competitor, recently (not Autodesk). The company that approached us to build a 3D customization tool on the web specifically required we use Onshape because they were a Solidworks shop and they were loving the web app's compatibility and features.
[0] It's hard to find a good name for what, exactly, it is. Storage-wise, a document in Onshape closely resembles a git repository. Under the hood, much/most/all? appears to be made up of FeatureScript, which is a language expressing transforms/operations on components within one or many documents (I'm not a cad guy/was concerned mostly with the back-end API, not the make-up of the docs, so this is probably partly wrong -- google FeatureScript for more info).
[1] That's not a back-handed compliment -- they were careful to manage expecatations about a major performance issue being resolved in code by a specific date. They often "hinted" at dates, and the dates they hinted at were hit, usually in advance. For the worst of the performance issues, they spent hours with us working out a suitable workaround that eliminated the latency.