As far as i know
If you're waiting and can wait a while, there is a recoil 417 in the pipeline.
how customisable are they? If you get a socom with the normal batteries then it'll take pretty much any normal external (apart from receivers and specific recoil outer barrels). It's a TM so normal TM compatible parts will fit.
how reliable? It's a TM, there's more to go wrong than normal aegs but it's not a problem. I've had two for coming on two years and neither have had issues. One was second hand and was running fine with an upgraded spring beforehand for another year and a half.
how well do the perform? Very well, some of the best guns out of the box and very fun to shoot with the recoil weight.
are they worth the money? Yes.