Ranked in these QuestionsQuestion Ranking
Pro Same rendering engine as Firefox
Iceweasel uses the same rendering engine as Firefox which is one of the fastest browsers around.
Pro External security fixes
In order to work on any system, Firefox Linux libraries contatin copies of all libraries despite what's installed in the system. While Iceweasel uses the system libraries only.
This way, when a certain library gets a security fix you will get that in the next system update without having to wait for Firefox to update their library tree and wait for their next patch to be released.
Con Updates are released a couple of days later
Since Iceweasel needs to wait for Mozilla to release a new update which then has to be merged by the Debian team into Iceweasel, it can take one or two days more for Iceweasel to get updates.