Disk to disk over a WAN link to another site is ok. I am not a fan of having all my backups online, nor of having all my backups on tape. I've been in the backup game for quite a while now and people keep saying "Tape is dead." I don't think tape is dead. Disk 2 disk isn't the perfect replacement yet. Tape write speed is a limiting factor to some degree, but if you're talking about backing up with something like LTO4 you're going to hit network transfer limits long before you hit tape write limits. Unless you've got 10GB Ethernet. So if you're backing up to disk over the network then you're probably not going to see a huge speed hit going to enterprise class tape.
Backups are covering several contingencies most places: accidental file deletion, intentional data deletion/destruction, and disaster recovery, and sometimes data archiving. Depending on what all your needs are you probably need your data further away than the next rack.
Disk to disk backup comes in many forms, from scripted or manual file copies, to virtual tape libraries, to disk storage managed by your backup software. The VTL and disk backup managed by your software have the advantage of providing offline backup that can only be managed via backup software, and thus aren't subject to user intervention, and less susceptible to virus or malicious user attacks. I am using a deduplicating VTL now. Previously I had a large chunk of SAN space managed by NetBackup setup as a Disk Storage Unit. Since my total storage in this arena was limited I used it for backups that had short retention and didn't grow much.
I am not aware of a d2d solution that is meant to have it's drives pulled out and rotated. Since most of them are using RAID across all the disks. Which means your retention is limited by your disk size.
I strongly advocate against online disk based backups. They aren't really secure against tampering, and they can lead to data inconsistencies. I know a lot of people especially here on serverfault are using them but I don't think this is the way to make disk backup most effective.
I feel the same way about breaking mirrors and moving disks. Most RAID controllers will take it fine, but it is a lot of overhead to rebuild that RAID set and in the mean time your performance will suffer.