Another (mostly) happy VSTS user here, also loving the CI/CD functionality.
I too have had builds break recently out of nowhere:
1. Related to NPM being upgraded on the hosted agent, which was fixed easily enough (after trawling through logs) by forcing the task to use an older version
2. SQL LocalDB connections stopped working, so integration tests couldn't run - this lasted for around 2 weeks!
I also ended up on the Microsoft support forums... unfortunately, responses on those forums from Microsoft staff are invariably late and of infuriatingly poor quality.
I ended up on the VSTS Agent image GitHub site instead, and raising an issue there was far more helpful.
VSTS is a great product, I just really wish there was a decent support option for when things go wrong.
I also wish Microsoft would actively communicate with impacted customers to notify them where there are outages.
Oh, and I wish the hosted build agents ran on better hardware - they are slow, even compared to the smaller Azure VMs. For example, we have a build that takes 14 minutes on the (paid) hosted agent, or 3 minutes on a B2ms Azure VM. Some of that is down to NPM and Nuget package caching, but even the actual msbuild and test stages are much slower.