I have one. It was my dad's until he passed a year ago.
They have serious cool points. It's a good rifle overall, and the quick-change barrel is neat. There's some little parts that you could loose, and frankly the Garand was a better rifle IMO. The Johnson could have been a better rifle, especially if taken as a "system" together with the LMG as a SAW. Johnson had a prototype that used a modified BAR mag that was in the conventional vertical position. The Army built up a prototype belt-fed Johnson LMG after the war.
Most of what you hear about the Johnson is just parroting the same stories over and over again. There's a great Collector Grade publication, "Johnson Rifles and Machine Guns" that's got real detail and facts. It's about $50 on Amazon, and is an excellent source.
The rotary mag is neat. Sure, you could conceivably dent it and have problems, but I think it's less of a potential problem than folks yap about. The bayonet's a joke, and the worst part of the joke is the price original spikes fetch. I ran across the original bayonet for my rifle when I was going through my dad's stuff. If I didn't find it, there's no way I'd have bothered even buying a repro. It's that useless.
The rifles were used in combat on Gavutu by the para-Marines (who never jumped in combat), and later on Guadalcanal when that unit was transferred there. They seemed to have worked out well. At least as well as anything else would have.
Logistics is a real thing in warfare, and the Army's production of the M1 meant that Marines were going to be carrying the Garand, and the Johnson was on borrowed time. "worse" "better" or "just as good" doesn't matter. Logistics dictated the Garand was it. There were never any Johnsons that were ordered directly for the Marine Corps. All their Johnsons came from the Dutch contract. So really, it's not like the Marines had ordered production Johnsons and then had to buy M1's. They weren't invested in the Johnson in any real way. They were just scarfing up anything that would work at the start of the war.
After the war, Johnson rifles were dirt cheap. They were sporterized by the bunch. You can run across sporters still once in a while, but many of the sporters seem to have been either "unbubba'd" or stripped for parts. Parts used to be pretty easy to get, especially since the LMG used many of the same parts. Lately, parts are getting scarce.
The rifles themselves go for stupid money. If you want a pure investment, I'd look elsewhere. The $6000+ price tag means you have a small market to sell it to, and you aren't likely to turn it overnight. If you just want a Johnson, then buying sooner is better. It's not like they are going to get cheaper. I just think for six grand, you could do better with something else for investing. You can't do much better for a WWII semi-auto rifle for coolness though.