I have used a neverReach, which attaches to the rear of the seat rails rather than the seat post.

 I have a xlab holder on my seat and have had zero problems in 1000's of miles of riding. put wire cages on it so you can make the bottles fit snug and it shouldn't be a problem. I've seen a few people with it mounted to their seat incorrectly or with loose fitting plastic cages which I can see causing a problem.

Lots of people around here hate the XLAB, but lots of people around here hate lots of other things too. I have the XLAB system, mounts to the seat rails, and I love it, holds all your gear pretty conveniently.

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Portside Athletics Blog

I also think the XLAB is great. Holds bottles, tires/tubes, etc. and I've never had a moment's trouble from mine in over two years. Another option would be an aerobar mounted bottle like the Aerodrink or PodiumQuest's Revolution. The Aerodrink is a pretty solid product - I can't speak for the Revolution - haven't used that one.

I have both the X Lab Flatwing and the Tacx seat rail mounted hydration system. The Tacx puts the bottles closer to the saddle and is lighter. I never had a problem with the X Lab.

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Suffer Well.

Minoura makes a really simple two cage holder that attaches to the seat rails - rather than the post.

I just purchased this for use on my new ride and it is really nice - sits lower than the X-Lab - which is also a good system. Makes getting on and off the bike with tall bottles easier - due to the reduced height and I think it is probably lighter too

I have a X-Lab for training and off season cause it's simple and I added a mount on the back for a flashing tail light in poor light conditions. If you use MTB cages you won't have a problem with bottles popping out. For competition it's the Nevereach all the way, a brilliant design, and well implemented.

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http://www.whydoesgodhateamputees.com/

I have the Xlab Flatwing and I really like it when I'm riding on smooth roads. However, I often ride on relatively rough roads (believe me it doesn’t take much) and railroad tracks. Because the bottles (the weight) are so far away from where it mounts, there is lots of bouncing on rough roads and train tracks are a free-for-all. There's an engineering explanation for this that slips my mind. Currently, I'm using the Profile cages, which I think might be part of the problem and if my new rubber band system does not work sufficiently, then I'm going to try different cages that hold the bottles more securely. If I abandon the Flatwing, then I’d also consider the Tacx system because the bottles are closer and that means less bouncing. My question with the Tacx is: 1) it seems to be really close to the seat, does your butt hit it when your riding whether in the aero position or upright?; and 2) Can you fit the C02 cartridge and adapter onto the back like the Flatwing? Finally, I’d try to Minoura system, which is amazingly simple, but I don’t recall ever reading any bad reviews. My concern with the Minoura is: 1) carrying other stuff; and 2) can you still put a saddle bag (even a small one) behind your saddle? In the end, I was admittedly persuaded because I think the Flatwing looks the best (yes, I'm shallow), albeit it’s functionality has left me re-considering. Still, I’m devoted to still trying to make it work since lots of ST’ers have offered great advice on fixing the issues many of us are experiencing.

Here is what I did to my Tacx on a P3C. I took a XLab CO2 mount and a Nashbar rear saddle cage holder and put them together:



The CO2 are right up against the bottom of the bottles.

Here is a side view of the attachment of the Nashbar piece to the Tacx holder without one of the cages for clarity:



I did have to trim the Tacx holder to fit around the rear loop of the P3C seatpost with my saddle in a forward position.

Here is the Nashbar part, I had it as one attempt at a rear mounted bottle cage. I just used the plastic piece, a pair of nyloc nut, and a spare bolt to attach the Nasbar cage holder to the tacx holder.

http://www.nashbar.com/...toreid=&pagename=

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Suffer Well.