I often get into discussions about "next-generation firewalls" versus "traditional firewalls". All firewall vendors are moving in this same direction, adding deeper inspection to their devices.
For sales, it's a very easy pitch. Everyone is annoyed with the limited visibility L4 firewalls have and proxies have always been a necessary evil. Along comes this one new box that will fix all these issues. Who wouldn't want one?
Marketing will say you no longer need to worry about TCP or UDP ports, just applications. But if you think about this for a few seconds, you'll know this isn't true. Applications can only be identified once a connection has been established and a certain amount of traffic has been passed back and forth. So if you want to allow HTTP on any port, you need to allow TCP connections to any port and block them once they turn out to be something different than HTTP. You just opened your entire network to portscans and all sorts of badness. This is a very easy configuration error to make - I've seen it happen several times already.
I was going to write about some of the interesting things you could do to trick the application-identification, but found an excellent presentation which pretty much sums up my thoughts:
DEFCON19 Presentation Slides
DEFCON19 Presentation Video
You may recognize the CLI - but this applies to all vendors. It's about the technology, not the individual products.
Does this mean application firewalls are useless? Absolutely not. You simply need to be aware of its limitations.
In my opinion, L7 firewalls are an additional layer of defense, not a replacement for the stateful firewalls. You still need a stateful L4 box in front to attach the network segments to that don't require application-identification or need better performance. This also reduces the risk of configuration errors on the L7 device.
On the L7 firewall you need to remember to only allow applications on the ports they need and that application-identification is an expensive operation, especially for certain UDP based protocols.