Addictive for sure!
I tend to do the same, wade in for a while then take a break and do something else. It works for me.
I've not used other sites for mine, ancestry is slick, the way they piece it together, search for links, other users can contact as per this post and how user-friendly it all is makes it difficult not to use them. Also, the amount of documents is incredible. So, I pay the money, although it's not the cheapest thing in the world I guess.
Yeah, I'll break down and pay for Ancestry one of these days. I was about to do so when I (re)discovered the revamped site of the LDS (revamped since whenever I saw it last anyway). FamilySearch.org has tons of documents and links to other places with them, all for free. I'm sure it's nowhere near as comprehensive as ancestry but there's a lot, especially census records, birth and death records, etc. They don't have the more obscure stuff you can find on Ancestry but it's enough to keep me busy indefinitely.
I've heard of others and that people can get a fairly good family tree put together with them, but as you say, it's the extras that Ancestry has and for those already a long way down the track with their ancestry, it's those extras that provide additional value. Do it when you want to, there's no point paying for it if you don't have the time to use it effectively.