My guess right now is it's not just you.
I'm getting STATUS_CODE_ERROR & an eternally spinning login…
… as are all these people - https://discussions.apple.com/thread/6868907
Google gives a lot of news hits for "app store down"
Doesn't seem to be affecting iOS 8.2 downloads - mine is coming through just fine right now. Thought that may have been related, but appears not.
Update 17:30 GMT - Apple Service Status now shows App Store as down - https://www.apple.com/support/systemstatus/
Update 18:45 - source The Telegraph
The problems are estimated to be costing Apple more than $4m (£2.7m) in lost sales from iTunes and the App Store. With these systems down, users are unable to buy new apps or install updates to ones that they already own. They will also be unable to buy music or videos from iTunes.
Apple has apologised for the problems. In a statement, it said: "We are apologise to our customers experiencing problems with iTunes and other services this morning. The cause was an internal DNS error at Apple.
"We're working to make all of the services available to customers as soon as possible, and we thank everyone for their patience."