Frankly (haha!), most franking machines are a pain in the butt. Everywhere I've used one it still dials up over PPP to top-up and you can happily top-up hundreds of pounds each time (I believe it just gets invoiced, but I don't really care).
The ink-cost alone is probably wiping out a lot of the savings of franking mail, not to mention the hassle of keeping analogue lines around, or buying the start-of-the-art Ethernet ones and supplying power/sockets/Internet to them.
To be honest, the largest use of them I see if to print pretty postmarks, not to save any money. And it's one hell of an expensive post-mark when the inks are nothing but an old HP deskjet cartridge with some knobs on to stop you cloning them (literally, I recognise the casing / head layout).
If I ran my own company, I'd say sod it and go to CostCo and bulk-buy stamps and stick them on. By the time you pee about weighing the item, feeding it through, correcting paper jams, replacing inks, topping up, etc. you might as well just lick a stamp.
And if urban myth is correct, bulk-buying of stamps is a good way to beat inflation...