I've now been to quite a few. They can be very hit or miss. When they are great, then they can be great fun. Much depends on the weather, and how tired you are.
The MK ones are or more value than the Epcot ones, IMO.
When the weather doesn't cooperate, then Test Track goes down. For Epcot, the main draws are Remy, GoG, Test Track, and maybe FEA. Many top MK attractions also go down in bad weather, like all the outdoor coasters.
If you get in line for any of those at the start of the extra hours, the wait is often still fairly long. Some people don't mind crossing Epcot in the pouring rain, but it is less appealing than when the weather is nice, just for Remy. If you want to go from FEA to Remy, you have to go through Mexico-Canada route. Much of the world Showcase is closed (edge of Norway all the way around to the edge of France.)
Periodically, we have also run into logistical problems if we go to dinner, and try to get back to the parks near closing time or dinner in Epcot runs late. Conversely, there is very little food available during the extra hours. The fish and Chips places sometimes stays open for a bit,and the main gift shop sells some packaged snacks. If you have a car, Miller's Ale House is open late, and they do carry out.
But then, much of what I just posted can also be said of the after hours. At least with the deluxe extra hours, even if they are a bust, you are getting the hotel.
Even when GoG is very empty, it still takes about 30min to get through the whole ride.
These days, WDW rides often go down. On my last visit, TT, GoG, Space Mtn were all down part of the extra hours. On the other hand, 7D was walk-on.
The extra hours are often a good time to hit the ILL rides. We've been able to ride TRON, 7D during the extra hours with minimal waits.
Much depends on how generally crowded the parks are during your week. If it is a slow week and a cold night, nearly everything will be walk-on, esp the last 90 minutes.
Much depends on your late-night endurance level.
When they are good, they can be a great deal of fun! Personally, I LOVE the parks late at night. I tis especially fun to ride Haunted Mansion after midnight!