We choose Cisco ASA 5500 Series for our branch office primarily because it is a stable firewall. Many home and even business grade firewalls will often start acting up and have to be rebooted, but the ASA is completely rock-solid. ASA Firewall Chains STP and RST Protocol allows us to build redundant uplinks to STP compatible switches. It has 256 MB RAM and 128 MB of flash which is plenty for future upgrades. I personally like to have the multitude of VPN options such as - IPsec VPN, DMVPN, L2TP, SSL, Any Connect, etc. The IPsec VPN is supported on the iPhone, so it is cool to be able to access my home network from my phone.
Room for Improvement:
Extraordinary learning curve, especially if you do not have previous skill with Cisco PIX or routers. Even using the Java-based ASDM, it can take time to find your way. In addition, ASDM is not compatible with the latest version of Java (you will get an 'unconnected sockets' error). No support for DHCP reservations. I like to configure Servers and Printers this way, and cannot find any decent reason Cisco would not support it as they do on their routers and Layer 3 switches.