I use it pretty frequently on aircraft. Generally, yes, it enhances a kit. I tend to use PE sets for cockpits, and especially seat harnesses. The exceptions are when I'm fine with what comes in the box. Sometimes all I add are the PE seat belts. But, truth be told, I prefer resin seats. Resin detail is better in my opinion and, to my eye, has a more natural or organic look, especially when combined with the typical raised detail in the cockpit. For me, there are few greater satisfactions at the bench than a well painted pit with some nice resin enhancements. Resin detail and raised details in the pit complement each other in a way that PE doesn't. This isn't to suggest that you can't get similar results with PE, but you may have to work at it if you're just stepping into using it. Where PE is great is for instrument panels, side consoles, various skin panels,exterior details and the inclusion of details that didn't come in the box. It's also great for things like emblems, pedals and grilles on car and armor kits.
They can get carried away sometimes, though, with all of those tiny little levers and whatzits included. I generally don't use any PE parts that have to be installed with the aid of an electron microscope, you might as well just drop those on the floor since the carpet monster is gonna get most of those anyway. One major problem with PE, at least for me, is working with it. Not only are some of the bits absurdly tiny, PE can also be difficult to work with, where bending is involved especially when something has to be bent just so, like afterburner rings and petals and the like. Then there are other things you have to fold together like boxes and even PE seats (which can be a mixed bag, depending on scale). Things like those, I just go with resin. The problem with PE is that it doesn't accurately replicate some things. It's fine for flat things like belts and IPs, but not for things like hydraulic tubing in landing gear bays. These are tubing and hoses, round not flat. So it's funny that PE manufacturers bother with it.
Is it worth it? Like you said, PE can be expensive. Not always, though. Sometimes you can get by with just belts or some basic pieces. I don't know what you build, but if you build aircraft some of the cheaper Eduard sets can be pretty cheap, depends on the subject. Some of the zoom sets are pretty nice, provided that they get the colors right. Nothing worse than getting a PE pit set, only to find it is in some weird shade that doesn't match your paint. For my part, I don't use much PE where it won't be seen. Other than the most visible spots like the seat, IP and consoles, what's the point?
Sorry, I didn't mean to write a sequel to Boswell's Life of Johnson here, but hope this sheds a little light on things for you. In the end, PE can be effective, but it can be tricky to work with at times. Just depends on the subject and how far you want to take it. Sorry for no pics but my present build is the Tamiya A-1J, OOB except for resin seat.