I have a new SP4 type cover with fingerprint reader, for my SP3.
The good:
- Fingerprint reader is FAST and was very easy to set up using Settings, Account, ...
- Backlighting seems to be improved. More evenly lit.
- Key action feels wonderful.
- Volume buttons, finally.
- Fits screen of SP3 just fine
The bad but not very bad:
- The extra fold magnet does not work well with the SP3, perhaps because it is a bit too long (left to right, similar complaint to docking station post, above). Must use keyboard flat.