As for battery life, Handspring says that the HSM's batteries should last for 3 hours of continuous use.  When looking at the HSM, one has to weigh which is more important: battery life or size.  Because the HSM uses its own batteries, it doesn't drain the Visor like other modems would.  If you plan on using a modem often, the HSM is probably better for you since you won't be needing to replace the Visor's batteries.  On the other hand, if you will be using a modem infrequently and size is more important, the HSM may not be the right modem for you.  This modem does weigh a lot and is clunky -- you will not be leaving this Springboard module in your Visor during day-to-day usage unless you really are online that often.

So what case do you use to carry the HSM?  The RhinoPak 3000 has a zippered front and back pouch that has more than enough room to carry the HSM and its cable comfortably.  If you will be in the elements, the OtterBox 3000 can carry the module fine, but you will need to carry the phone cable in your backpack as there isn't enough room for it.  If you need something smaller than the RP3000, the module and phone cable fit nicely inside the pouch on the front of the RhinoSkin RhinoPak 2000.