My cheap wired Bell brand computer measures within a few hundreths of a mile on the same route every time, well within any minor distance variations I would expect from trip to trip. On a day I'm getting good GPS reception, it is usually within 1.5% of the GPS (I have a voice that announces distance every 2 miles, so I can look down at the trip meter).

The biggest difference I notice is that while the computer is always spot on, my GPS reception can vary from day to day. On my normal after work ride, I've had days where my outbound leg reads 8.0 and the exact same inbound leg reads 7.6, where other days they read spot on with one another.