It all depends on the system in use.
We receive mode S data and see the callsign which has been input, the selected Flight level, Mach No. Magnetic heading and the
IAS 
The selected flight level is very useful and has stopped a few level busts !! Although some pilots can be very surprised when they realise what information we can see.
Apparently a lot more information is downlinked, but is not shown to us lowly controllers
