You get their filtering power for free and don't have to deal with the
hardware, if you don't particularly like it.
That's not completely true; the Barracuda appliance uses both block-lists and content-based filtering. The block-list is free for anyone who wants it, but the content-based filtering is not.
However, the block-list *is* now one of the best ones out there. They had a rocky start, but in the last year they have consistently outperformed most of the other no-charge block-lists both in terms of catch rate and false positive rate.
Spamhaus has long been one of my favorites for its performance, but I am now seeing Barracuda beat them each month in catch rate, sometimes by a nice margin. (FP rate for both lists is about the same; VERY close to zero.) If you like Spamhaus, you should try Barracuda block-list and see if it helps in your mail stream. (Every stream is different, so my results may not match your results.)