I read a lot of complaints on download.com that it has to stay in learn mode because it keeps forgetting its settings. I also heard it didn't hinder bandwidth like zonealarm, but another person said it did, perhaps they were referring to the annoyance of having to tell it what to do all the time.
Memory problem? I'll try and read between the lines to make the connection between your topic title and your post.

Earlier versions of CFP could use a bit more memory than users expected, but this has improved markedly with the latest version. Current usage is around the 22-28 MB mark, but this will vary depending upon your actual system and the installed software on it.
CFP does not have to stay in learn mode to remember its settings. This may have been the case with version prior to 2.0 but it has been stable since then.
CFP is quick - very quick. There is a slight drop in performance, but adding any additional layer to your communications will induce some level of lag, but CFP is right up there with the best.
You will get more alerts in the first couple of days, but you would get this with any decent firewall. The alerts drop off once the firewall gets used to the apps installed on your system. CFP's built in database of over 11,000 apps goes a long way towards reducing the amount of alerts you get (providing you choose to not show alerts for approved apps).
Hope this has covered it,
Ewen :-)