Our experience has been great. We're an ecommerce hosting provider with our own AS. We were looking to scale up to 20G and far beyond while having some ddos protection. AFM/ASM filled that role better than any of the dedicated ddos equipment, traditional firewalls, or next-gen firewall providers could.
We currently have over 2,000 dedicated servers behind a ha pair of viprions on 11.3 and they're running great. We've pushed them up, and over, their stated new connections a second easily. Just make sure you set connections per second and max connections on your virtual servers. Otherwise they will try to process everything and overwhelm the cpu; resulting in lot's of connections being reaped. The reaper is what keeps them from falling over like traditional firewalls do when session state tables fill.
Overall they're amazing devices and perform as stated. They are layer 7 proxies with high performance hardware. You can tune and inspect everything imaginable. It's overwhelming at times but the support people have been really helpful with it.
They are ICSA Labs certified firewalls and are doing quite well in all the vendor tests being run (see the latest networkworld release on the f5 homepage).
We're going to be doing a video recording of our case study for f5, in several weeks, about our experience. So if you have any specific questions, feel free to ask, and I'll answer them as best I can.