1) Almost all of the FSX software runs on Prepar3d. I only lost one minor airplane.
2) Using MigrationTool you can install almost any FSX software. MigrationTool will fake-out the older install software & redirect it to the P3d location. Some updated FSX software will prevent MigrationTool from working. That is their right. I respect that. I just don't buy their software.
3) The settings are almost identical to those in FSX. Learning the differences is very easy.
There are, of course, many other changes. But what it amounts to is that, for the most part, you can keep and use what you already have.
For instance: P3d doesn't come with a Cessna C172. I simply copied it from FSX & pasted it into Prepar3d - voila.
Is it bulletproof? Nothing in life is.
On the other hand, as it's being improved, P3d is evolving away from FSX. Right now it's still a 32bit program, very similar to FSX. With Prepar3d V3.x things may change drastically.
Think of it as going from FS9 to FSX. Some things stopped working, and you needed a more powerful computer.
The same will probably happen when Prepar3d V3.x comes out.
The biggest negatives
To some, Change, is the biggest negative.
At the moment, there is a radius around the aircraft that is sharp & clear. Scenery beyond that radius is blurry and will "pop" into clarity as you move towards it.
As I have confidence that they will fix the problem, I simply ignore it.
To others, it drives them crazy.
In short, there are many small improvements over FSX which I think make it a clear step upwards.
From the standpoint of many FSX users, I don't think that they'll see any compelling reason to switch. Yet!
But it's definitely on the edge of "compelling".
The cost of Prepar3d
Think of it as six or seven trips to McDonalds. Or one and a half trips to my wife's favorite Mexican restaurant.
You get the idea.
But whereas restaurant meals come and go (usually the next morning

Coming soon
Just about every software maker will charge extra to upgrade from one level to another.
I expect that LM will do so when Prepar3d goes from Version 2.x to 3.x.
Possibly this year, but maybe next year.
But if Prepar3d V2.5 satisfies you, there's no reason that you need to upgrade.