The Cisco ISR is a fairly versatile piece of equipment for certain telephony and networking applications. It's a good jack-of-all-trades type of device for speaking lots of different protocols and hardware interfaces.
I have a Cisco 3845 and a Cisco 2511 in my network, and I previously used an IAD2432 for some of the functions I now have the 3845 handling. The 3845 more or less functions as a tandem in my telephony network, acting as a ISDN call router and TDM switch for the various PBXes and telephony equipment I run. At present time, I've managed to find use for all 6 of the voice T1 ports installed in it, with potential plans to use more.
Similarly, my Cisco 2511 lets me have another IOS device to play around with for networking. I have a data T1 connection configured between the 2511 and the 3845 which currently only does IP connectivity, but could be used to play around with other protocols some day, like X.25. The 2511's 16 asynchronous serial ports also make it a good device to run my UDS modem bank and the AUI port lets me run a small 10BASE5 segment routing back to my main network over the T1.