Avast firewall isn't in the same league as some other firewalls in my opinion. And it definitely isn't as strong as comodo firewall. But, with Windows firewall being enough if you're behind a router I guess it comes down to how strong if a firewall you feel you need to suit your daily computer habits.
to me it's good enough than not having any, I'd rather prefer windows firewall with any other third party realtime protection module like Avast Free or so.