While there are POC exploits, it would be difficult to actually use them effectively against a target computer.
Difficult, but not impossible.
We need someone from the NSA to post here.....
Meltdown paper describes working exploit demo against live Amazon cloud server returning 500 Kb/sec from protected address space. To be good cloud citizens they did not breech other tenants’ memory but could have. This is NOT just proof-of-concept!