I've been using CloudControl PaaS for about a year. Their MongoDB support is nowadays actually provided by MongoLab, which is nice.
Pros:
- Using source control (git/bzr) to upload/update apps was weird at first, but it's actually quite nice, and if you use git or bazaar for version control anyway then it's totally nice. I use Tortoise Bazaar on Windows.
- Generally Just Works (tm)
- Customer support has been exceptional
- Scales easily, and starts from free.
- Very easy to use once you learn the basic tools and concepts.
Cons:
- Documentation is a bit sparse
- Their online management tools and website are... I wouldn't say very well thought or complete.
- PaaS limits, like no filesystem access etc. which might or might not be a problem
I can recommend that. Of course a good VPS is always good option, but might be pricey if you want to scale up easily.
Regarding the "any webhost + mongolab" or similar option, I think for most apps it is beneficial to keep the app server (PHP) and mongodb close to each other so that the communication between them does not introduce latency to the application and keep PHP execution times low if it needs to access MongoDB much. In VPS you can have them on same hardware, which is of course very fast. In the case of cloudcontrol (+their mongolab system) I think behind the scenes they all run on Amazon AWS Europe datacenters (which I think might also mean it's a bit slower to access from the US, but not sure if it makes much difference).