My expectations were very low, fearing boredom, repetition and prepared answers to the expected questions.
Instead, I found it constructive and filled with thinking on the hoof to twists on forecast questions.
It was as if they were given a script, but they then had to extemporate as the debate progressed, and that made it interesting because it always gives the opportunity for error, misstatement and outright abuse.
They became slightly unguarded at times, but no-one landed the killer punch, BUT I think that the next two debates will be even better, as they know the grounds on which they will be attacked, and those attacks on opponents which are successful, and that will allow better targeting. And more opportunity for vicious unexpected questions.
The debate winner: Brown, I think on almost every level, probably because so little was expected from him and he avoided a televisual suicide. The best debater was Clegg, but he won't win the election (emphasis added and updated after first comment). But Cameron has done enough to win overall, by avoiding anything any mistakes and by building on a satisfactory standing by making the right noises, even if they were intellectually weak.
Update: as I type ITN are carrying their poll of candidates. The low score for Brown should be compared to where he started from, which probably means that the Labour result will be slightly better than I thought some time back, by encouraging some disaffected Labour voters to actually turn out and vote. But I still think that Cameron will win the floating vote and will still win the largest number of seats in the election.