I felt it was a decent send-off, not a great episode, but also not a bad one. It was strange because it didn't feel like a finale until the final minute or so, it was just a normal (albeit extended) episode. I was wondering how they were going to end it, and for a while there I began to worry it would be some weird 4th-wall ending where the characters realised they were characters in a TV show. But the actual ending fitted the show just about perfectly.
Really? I thought the first series was by far the weakest of the bunch.
I think the first series was the purest of the bunch in that it was actually about the tribulations of an IT department. The rest of the series moved away from that and it became a more generic sitcom with nerdy humour, and later series were increasingly dominated by Douglas. I personally think series 2 and 3 were the funniest, but I can understand why some would prefer series 1.