They are seriously challenging. Challenging to the point that you will find it hard to complete with randoms via match making and no comms.
Its much less frustrating when you're in a crew with others on comms.
The pay outs are decent, we did the first proper heist last night and got 100k each. As leader i got 160k, but that was because I had to pay 40k just to start the heist. And there was a nice first time completion bonus of 150k or something. They aren't the most efficient way to earn money, as griding easy missions is probably the most productive.
But the heists are definitely the most fun. They have all the feel of heists in single player, but with your mates. Top work rockstar.
Only downside is how busy the servers are. Give it a week however and the numbers will be back to normal.