Yeah IPB is pricey but has some good features and is very good in maintenance wise and security. I got an example of a good IPB
http://hadzz.com/ (they just copied xbox.com's theme haha
)
Also one for MyBB
http://www.moditup.net/
anyways IPB its pretty good. PHPBB i had some really bad luck with. I had a very good host but the site got hacked in and basically destroyed but spammers and people accessed it. Not sure how may have been the host but i know many security walls where breaked in. And spammers + advertisers.
XenForo is very good, my friends have some (Still in developpement) and it works and is very good. Plus price is reasonable.
Theres also VBulltin. Pricey, but is excellent. I have one it has never had a problem. Ever and works perfect. Just on the price side. Has many support sites and very good support. (or for me anyways i got good luck) also many many themes some free that are nice some cost but nice also. SMF is pretty good as stated but i prefer paid things because you get more support better everything haha and upgrades etc. Latest security protect. Also forgot about MyBB, its very good its free and has nice features. Its similar to SMF and IPB merged.
So in the end the best i would suggest for paid is
(Expensive, new owners who are retarded and its now a shit product) VBulletin (Not recommended)
XenForo
IPB
Free ones i recommend
SMF
MyBB
I would suggest going with these other then Phpbb to prevent all the worst and dont take the chance. So mine as well take the one more reliable and its free.