KJW
M700- clone of the more expensive Tanaka model, so there's quite a bit of parts commonality when upgrading. The striker can wear down over time (and we're talking very heavy use) and the G&G steel replacement/upgrade is too durable, as the stock striker plate on the magazine can be damaged and worn down by it, meaning you'd have to spend about £40 on a steel replacement.
The Takedown model flexes slightly because of the join, and I have read about them having an inconsistent seal between both parts.
Mags can leak sometimes, and usually around the fill valve. Best solutions are to either tighten the valve (this can wear the valve's head down if you use a sharp screwdriver) or buy new O rings. I bought a 419 piece set and the 2nd smallest size fits the long mag perfectly, the rubber's just a little thick for the short mag.
Barrel- stock barrel is ok at best. I'm hitting man sized targets most of the time at 40-50m and I've hit targets further away than that at skirmishes. You have to watch your suppressor adaptor though, they can fall out veeeeery easily and wobble about if not fitted properly. I'm fitting a 6.02mm tightbore next week and I'll be experimenting with that.
The hop unit is a pretty odd design- it's operated by twisting a ratcheted disk, which screws down a thread and puts pressure onto the wedge shaped nub. Mine needed a little TLC and sorting out to get the best performance, and I'll hopefully be making more with a 3D printer next year in various designs (PT247 I can offer you 1 or 2 if I manage to perfect one).
Judgement: reliable if you maintain it and clean it periodically if you use green gas, the silicone in it wreaks havoc on the hop rubber and other seals.