The primary thing to realise is that IT Support services - although being very useful to your business are also subject to being multi-leveraged services - so the noisy wheel will inevitably get the oil!
Reporting is often insufficient and controls may be sloppy - is your backup working?
Suppliers - a key element to a decent IT support company - do they have all the solutions available - no-one teaches them how to be an IT support company and only the best join together into communities where they can support each other.
And remember - the engineer mind may not translate well into social situations or dealing well with customers - i.e the need for a translator for IT speak!
In short you may need to take and ensure you have control of your business risk when dealing with your IT support company - be they a one man band or a large faceless corporation.