My girlfriend and I both like our NeverReach systems, and currently both plan to use them during IM Lake Placid figuring hydration is more important than the weight penalty.
Mine weighs 16oz empty according to the kitchen scale. That is the biggest downside. On the other hand, my old Camelback H.A.W.G. also weighs about 16oz empty according to the same scale.
The second downside is that I have not figured out any way to tell how full it is while riding (until it is empty). If I jump off the bike I can instantly see the water level. However, while I am actually moving, I can not see the water level. With a Camelbak I could reach an arm behind and press against the pack to get a rough idea. I have not figured out a similar trick for the NeverReach.
The third downside is that it changes your dismount a bit. You either have to really lift that leg, or mount/dismount tandem style.
The fourth downside is that you can not simply lay your bike gently on the ground when you stop, the water would pour out of your NeverReach. You have to lean your bike against something.
Refill is simple, just pop the top and stick a bottle in, the bottle drains into the reservoir. You don’t need to fully remove the bottle cover, anything that will let the bottle drain when upside down should be sufficient.
Emptying after a ride is a bit trickier. I usually just invert the entire bike! I definitely do NOT remove the thing from the bike very often. Because of that I only run water through it.
I found a little saddle bag that worked with it and my saddle setup, your results may vary. However, check out the 50 cu. in. Trek “Pro Pack.” You want the one that uses only velcro to attach to the saddle, not the one with the quick release cleat.
If you like to drink in the aero position, you’ll love it. If you like to drink from the hoods, you’ll want to leave a lot of extra blue tube above the black tube cover, and the tube will flop around a little. However, it is still tolerable. The black tube cover contains a wire allowing aero position drinkers to locate the mouth piece wherever they want. However, I run my tube so long the wire is not strong enough.
Service has been excellent. I pre-ordered ours as an X-mas gift. They sent out the system when it became available. Later with no prompting from me they sent out a “solid cover” alternative to the splash guard cover. Later they sent out a hose clamp to address a slight drip problem, again with no prompting from me. Recently they sent out decals, apparently now they have time for more marketing. 
We generally ride with the “splash” cover and a bottle in place, but put the “solid” cover on for storage and travel. I have not noticed any problems with water splashing out during a ride, unlike my brief experience with a Profile Aero Bottle.
My girlfriend actually plans to fill hers before the start and avoid the IM water stops. I’m leaning more towards slowing for water stops and running my NeverReach close to empty to save weight.
– Bill – Lake Placid IronVirgin – Starting to get pre-race jitters 