I agree with Robertsr... a Pneumag is the next best thing to sliced bread! The nice part is you can make just about any Mag a Pneumag. All of the components are in the grip frame and you can opt to put an LPR in the grip too or have it external. I have seen a few Pneumags sell, but at times they are too much. I think there are a couple out on AO right now and one in particular is a sweet setup, but they are asking $800! In my mind that is too much for what they have in those. I would assume that a fair price would range from $400-$600 for something completely brand new or in like new condition. Something used or well used would go for about half that. Again, these are basically a mech Mag with pneumatics to make it shoot faster and to give you a super small and light trigger pull. A lot of the cost for one of these will stem from what it was built from. Remember, a new ULE Mag will run around $400+!
The E/X-Mags are completely different than any other gun I have ever owned. I love the way they are electric and then you can flip a switch and they are mechanical again. I loved the E-Mag I had, but my X just makes me
They (E/X-mags) are heavier than standard Mags unless you buy all ULE components and then you are still around 3 lbs, which isn't bad if you are used to lugging around a VM. The E mode will allow you to walk the trigger, but mech mode you won't be able too.
FYI, Mags are not low pressure. Use HP air sources and large ones too. I shoot a 68/3000 and get around 700+ shots on average. A 68/4500 or larger will obviously get more. Adjustable output tanks are nice since you can crank up the output pressure to get more of a reactive mode on the trigger, but be careful because it can cause the Mag to "run away!
Overall maitenance is simple. Infact its just as simple as taking care of PPS guns. Usually some oil in the ASA and gas it up followed by firing a few shots to cycle the oil through and you are good to go. Tear down is accomplished by removing the field strip screw that holds the valve on and then sliding the valve out. With a Pneumag, you don't have any more maintenance, but you may have to replace the LP hoses like you would with a cocker and possibly work on the LPR. The maintenance for the Mag's valve is simple. Generally a $10-$20 seal kit will fix all your issues if there is a leak, but as long as you keep some oil on the seals it will last you for a long time.
What kind of Mag to go with generally depends on your style of play and if you want to use CO2 or HPA. Mags work great on HPA, but you can only use CO2 on classic valves. X-Valves are only recommended to use HPA, but a classic valve can use both. ULE components are wonderful, but limited because there aren't very many sources for aftermarket parts. Also, newer ULE bodies are cocker threaded for barrels, so if you don't want twistlock then go with a ULE body, Doc's barrel adapter, Micromag, or an X-Mag so you can use cocker barrels. Another nice point is you don't need a secondary reg. Running an airline straight from the ASA to the valve works the best if you are using HPA.