I didn't enjoy the humour quite as much as the last two games but the Toybox gives a new dimension to the puzzles that keeps it fresh. Also has a much stronger dramatic tone that reaches it's peak at the ending which was cool to see. I prefer the more relaxed vibe of the games that came before but it's good they mixed it up a bit.
I'll never forgive myself for not pre-ordering for Max's Severed head in TF2 and getting a nice payout years later.