I think it depends heavily on what area you want support for. EC 2 support is generally very good, billing support is reasonable if a bit slow. Support for a lot of their managed services, or media services is beyond useless. All the support engineer does is take your complaint and says they will check with the internal service team. Your ticket will just end up in "waiting for Amazon" state for weeks or months.
It's been 5 years since I was responsible for anything AWS, but back then at least support was surprisingly good even when spending less than $100 a month. Emails would get answered and I could often get someone on the phone within 24 hours if I needed.