Hi
It really depends on what you are trying to achieve with your firewalls.
Multi mode is useful if you have service provider type setup where you can allocate a context to each customer and give them control of their own virtual firewall. It can also be useful if you have different depts. within your company which are responsible for their own security.
Having said that we use multi contexts on our firewalls in our datacentre. It allows us to segregate the firewalls based on server function which makes the access-lists more manageable and we can also create a context on the firewall which maps to a context on our ACE blades.
There are however some downsides to using multi context which may or may not be an issue for you.
1) The context licenses themselves are not cheap as you are in effect buying multiple firewalls.
2) You cannot run a routing protocol on the FWSM's. In single mode you can use RIP or OSPF on the FWSM's but in multi mode you can only use static routing.
3) We are currently running v2.3 on our FWSM's which means you cannot have a mixture of routed vs transparent contexts. I believe this restriction has been lifted on v3.1 but it's worth checking.
Overall i'm comfortable with the decision we made and haven't found any of the restrictions too onerous. What i would suggest is that you work out how much firewalling you are actually going to be doing in terms of access-lists, statics etc, who needs access to the firewall (is it under single management or not) and if you are planning to deploy any of the other sevice modules.
Any further questions let me know
HTH
