"Any good" is kind of a question that depends on how you are defining "any good".
Does "any good" mean "capable of providing reliable, satisfying performance over the long term?" If so the answer is yes. They are great for durability, simplicity, economy, low cost of operation, very pleasing smoothness and sound, and very good power *within the realm of what it is*. Your engine has over 500k and is still running even if it's a little tired out.... plus it probably spent most of its early life running on poor quality conventional oils rather than modern synthetic oil that most of us use in them now. So I would say if it has done all those kilometers, it is certainly a good engine.
Running on modern synthetic oil they last more or less indefinitely IF of course you don't try to kill it by overheating it, and give it basic normal upkeep. So if you overhaul it and then take reasonable care of it, you can expect to get lots of good service from it.
But if "any good" means you are asking "is it competitive with a much larger, much newer 24 valve OM606 Mercedes engine in terms of performance?"..... then the answer of course is no. If you are seriously going after 300hp or more, then swapping a different engine will be easier and a more successful long term proposition. People have done 250-300hp out of the D24T platform, and similar kinds of specific output out of the close relative 4 cylinder versions of the VW IDI engine, but it is not common or easy. With the 606 engine of course it is easy, with stuff you can buy off the shelf.
D24TIC is stock around 125hp, can easily reach ~175hp, but doing more than that is pushing the limits of the platform. You only have 2.4 liters, 12 valves, and 1970s combustion chamber and fuel system design to work with.
They are good and trustworthy engines within the bounds of what they do well, but if you want it to be something it is not, then you might not be too happy in the end.
The power curve you are describing sounds about right if you have a manual transmission. If the engine is worn out then that is probably contributing too.