I've used ZoneAlarm for over 6 months now since I first got the internet in at home.
I've never noticed anything out of the ordinary regarding memory usage - and it seems to be protecting my PC completely. Of course I kind of suspect that the 30-40 or so 'hack attempts' I get a day are probably just bull, made up by the program to show me its protecting me. I'm a cynic, I know. I briefly changed my firewall to another free one, and didn't get any hack attempts, but I changed back out of fear that maybe they were real hacks, and this firewall didn't stop them - plus it wasn't as nice looking as zonealarm. Anyway - you can turn off all the hack messages, so thats fine.
Its very easy to manage all the programs that want to access the internet, recognising programs that have 'changed' and asks again if you want to allow it access.
Only thing I don't like is when I update it, I have to allow all the programs access again - it should retain that info.
