I'm not an expert on all things backup -- especially when it comes to the mac, but from what I can tell, the Airport Time Capsule (nor any NAS or external hard drive for that matter) won't be able to directly back up your iphone or ipad devices. It may be possible to do so through itunes or something, but would likely require some manual effort to get that set up. Once again, not an expert here, so if I'm wrong please someone correct me...
Additionally, the ATC is a single drive device so it doesn't have any sort of redundancy of backed up data, so if the drive in it ever fails (and they all eventually do), your backups will be lost -- depending on how you plan to use it, that may still be sufficient, but it may be worth looking for either a NAS that supports a RAID configuration (at least 2 drives that each contain a copy of the data so if one drive fails, you can replace it with a new drive and it will rebuild the 2nd copy) if you want something a bit more robust.
Last but not least, there are several offsite solutions that may be worth looking into as well -- Offsite backups have some benefits such as letting someone else do all the work of maintaining the hardware, and if something tragic like a fire, tornado, etc. destroys your computer, your backups will still be stored safely off site. In the case of something like irreplaceable family photos, this could be of critical importance -- As I said, it all depends on what you're looking to do.