I also think this is a good setup. I currently do run a Raspberry Pi, connected directly with a 3.5'' USB 2.0 hard disk (Western Digital, 1.5 TB) with external power, and it works like a charm: I get hourly backups when I'm home, and the Raspberry is also an excellent file/media server.
I get 23MB/s instead of 30MB/s on the hard disk, but that's still quite good.
I can stream movies off the hard disk (through Samba or MiniDLNA) without problems.
In fact, the hourly backups are done from my laptop using Apple Time Machine, but using
rsync shouldn't change much.
As @goldilocks mentioned, hard disks without external power (like most 2.5'' hard disk) will require a powered hub: my Raspberry shuts down as soon as I try attaching one directly (edit: that was with a 5V-1.2A power supply, but still happens when attaching directly with a 5V-2A power supply; a powered hub solves the problem).
In addition, I've installed BarracudaDrive (http://barracudadrive.com/), available for free from the PiStore, which I'll use as my personal cloud in the future.
Hard disk reliability
The USB HD powers down when not used, but this is implemented by the HD enclosure (most recent ones I bought have this).
Note that spinning down and up is a stress in itself for the hard disk if done too often, but since you can't set the spindown timeout, this is only a problem if the producer chose the wrong setup.
(Google for "ubuntu destroys hard disk" to find discussions such as: https://ata.wiki.kernel.org/index.php/Known_issues#Drives_which_perform_frequent_head_unloads_under_Linux).
Anyway, you should simply use a reliable USB hard disk designed for your kind of scenarios. Or two ones in mirroring (edit: but I would check performance first, since mirroring would double the required USB bandwidth for the same amount of writes).
Instead of mirroring, I carry a rugged hard disk always with me and do backups also there.
General software setup tips
Of course, you should pay attention to the filesystem you use on the HD: nowadays ext4 is probably the safest choice since it's easiest to recover it.
You might want to use
rdiff-backup instead of
rsync, to have access to historic backups.
Finally, since you'll probably do rsync over SSH, you might want to select the RC4 cipher for extra speed (edit: but see the warnings about security in this other answer if you do that over the Internet). Here's an excerpt from my .ssh/config on my host:
Host pibbw # Replace pibbw with the hostname of your Raspberry Pi