Its so rare for there to be a topic I feel knowledgeable about on here! (compared to the vast number of far more experienced people). Anyway:
I owned 2 in college. An '84 solid rear axle (only year for that) with the CA20E, SOHC, dual spark plug per cylinder mystery, and an '85 IRS FJ20E grey market import from Europe.
The '84 was bought for $400, but ugly and with wiring problems. A full clean up and some re-wiring, removing the 80s-tastic hatch louvers and chrome hub-caps (you read that right), and some porterfields and azenis made it an entertaining momentum car for a college kid. Went from this:
to this:
and I eventually blacked out the window trim:
Pardon the door dent, the previous owner was kind of rough on it. All told I had about $1000 into it (including purchase), and sold it to a kid in the area for $800 when I graduated. No complaints.
The '85 I bought from a guy whose parents brought it back from Europe and gave it to him (i think as a first car), and he drove it for a couple months and blew the headgasket. Got it cheap, didn't realize how hard it is to get parts for a grey-market car and motor never sold in this country, and sold it unchanged for the same $$ like a year later. If I had it now, with a garage and decent job, I would have fixed it up, but a college kid's time and budget was better spent on other things.
Cool car though.
All that being said, I think the reason you don't see them more often is due to a couple things. As mentioned above, the 240SX is a better car in every way. Better suspension, better engines, better brakes. Also, parts availablity, even for the non-grey market cars, is terrible. All Nissan OEM parts for them (and most other Nissans, FYI) was sold to OEM surplus, a company in Michigan? (I think). They're actually great to deal with, I bought a wiring harness from them to fix the fuel pump issues, but as the parts are bought up, availability suffers. Even simple things like rubber hoses and wheel bearings from parts stores typically was a mix and match affair, requiring either cross-referencing to 280Z/ZX/300Z, or bringing the old part in and sizing. When I had to replace a hub, there were NO new ones available anywhere, and I went through 3 used ones purchased online before I got a rebuildable example.
I finally decided the time and aggravation weren't worth it when there were other cars that were as enjoyable or more so for the same money (E30s, Miatas, MR2s, etc.)
Couple of nice things though:
-All common Nissan motor swaps go in just fine: CA18DET is practically bolt-in, KA24DE, SR20, even VG series motors.
-You could get Konis for them when I owned mine (~4 years ago), I think they were Z-Car part #'s (280ZX for the one axle, 300ZX for the other if I remember right)
-Porterfield makes brakes for them
-If you love cars with a strong 80s aura, this is right up there with Starions and C4 Corvettes for that, mine had a talking lady that told you to buckle up, when the doors were open, when the lights were on, etc. The grey market car even had the initial-D "chime" above a certain speed
-The seats on my '84 were some of the best OEM I've ever had. They were comfortable, aggressively bolstered, and had lumber, bolster, and thigh adjustments. 8 hr road trips in that car were NO problem
-The back seats folded flat and were level with the trunk, and the rear quarter windows could pop open. Great for camping and sleeping at the track.