I have an Iomega firewired to my iMac. When I bought my (late 08) macbook, I moved the Iomega to my airport extreme base station, via USB. I found it horrible for two reasons: 1) way slow and 2) it created a duplicate backup folder for the iMac. I never even bothered to set it up for my macbook.
So I disconnected everything, deleted the mirror backup, and put the Iomega back in the configuration I had before. For my macbook, since I don't use it as often, I'll wander upstairs and hard wire it (USB, no firewire this time) to the Iomega. It's a slight pain in the ass, but it works perfectly fine.
Had I NOT owned an external HD, I might have considered getting the TimeCapsule. Doing it via USB to airport station, while technically workable, is not worth the hassle for more than one mac needing backup.
My 2 cents.