I purchased one of these for my company as a router/firewall, and to provide Site to Site VPN to another 5510 at our hosting provider.
The ASDM GUI is very easy to get working (plug into the management port and browse to
[...], but to really configure this router, I would recommend being comfortable with IOS and the command prompt, and really really proficient with networking concepts.
In my opinion, if the following looks too hard, you should seek another router:
hostname(config)#access-list OUTSIDE extended permit tcp any host 172.20.1.10 eq www
hostname(config)#access-group OUTSIDE in interface outside
If you are a command line god, and you really understand networks, or if you have one who can teach you, then this is an outstanding router. You can do a lot of config from the GUI, but you still need to understand the concepts, and you will likely find yourself checking the running config after making a change in the GUI just so you can do it from the command line next time.
In short, if you are a Cisco guy or gal, this particular router is great, and inexpensive. It seems to scale well, too, but I don't have hard numbers on that yet.