During winter testing there was a lot of discussion that how presumptive the winter test results really have been. So let's see:

Ferrari and McLaren were quick and they dominated in Australia.
BMW also looked like a strong force. A couple of persons said that they are the team to beat, but that was understandably an overestimation.
Renault acknowledged "need for speed" and so it proved.
Williams looked fast and a candidate for points.
Honda and RBR looked bad throughout the winter.
Toyota maybe looked a bit better than expected, but they were also often in Top10 in winter tests except the last common test days and they had an extra test at Jerez, where they were already better, but couldn't be compared to others then.
Super Aguri also looked fast sometimes.
The team that "fooled" during winter a bit, was STR, who was in Top10 at Sakhir, but was way off at Melbourne.
And Spyker looked like the slowest.

So in conclusion it can be said once again: if you don't get too excited about lone fast laps (like RBR's fastest lap at Barcelona's last test day), then winter tests give a general picture, what can be expected from the start of the season.