NSB hasn't existed for 5 years or so now. SJ run the trains to Trondheim, and onwards to Bodø, and all the regional services from Trondheim. Vy, who are effectively the continuity state operator, run the route to Bergen and everything in the greater Oslo area (including trains to Gothenburg and Skien), and GoAhead run the trains to and around Stavanger.
In general they are still all using the same stock (almost all of this was new in past 10-15 years if you haven't been for a while). The Bergen route has gained some business-class style fully-flat seats on the night train though. Overall I'd say it's fine, and in fact the standard class seats are good - reasonably wide, good legroom and some level of recline. They also pretty universally have tray tables which can be pulled out to accomodate a laptop and power at the seats.
Norway fundamentally doesn't believe in first class though, so the best you'll do on most trains is a standard seat with free coffee. The free coffee is not good.
In terms of ticketing, everybody is using Entur for fulfilment. It's also used by a growing number of bus companies, and gives a really impressive ability to buy and manage all the tickets in-app (or via printable pdfs), as well as timetables, journey planning and live updates. Unfortunately GoAhead have a loyalty scheme if you book direct, and Vy sometimes seem to have better offers if you book direct, so you can't safely just use the Entur app/site for everything.
Prices outside of zonal areas are unfortunately quite high for advance purchases and absolutely scandalous for walk up fares, so you really need to plan ahead.
As you mention, journey times and frequencies outside greater Oslo aren't fantastic because there's a lot of very bendy single lines. This also makes reliability a problem, and connection guarantees are all a bit uncomfortable between the different operators. Most of the staff don't really know how to help if a connection is missed, although as far as I can tell they basically also don't care if you just get the next train, but of course that might be the next day.
The network is developing more quickly than anywhere else I can think of though, with new double track into Bergen just opened, big improvements into Drammen recently, a new lengthy tunnel to Ski and Gothenberg opened a year or so ago, and a long stretch of double track from the airport to Hamar via a straightened route which has been opening in stages and continues. Upcoming plans include further work north of Hamar towards Lillehammer, a tunnel even further out of Bergen along with more and longer loops further out, and double track to the Swedish border towards Gothenburg.
These interventions all tend to knock a casual 20-40 minutes off journey times and hugely improve reliability. I'd say that Bergen is probably now just about competitive with the plane, and certainly once they've finished the next tunnel it will be.