The first time I rode with my bottles mounted to my seat instead of having them sitting in racks attached to the bike frame I felt as if I'd discovered something magical. If you are a true speed geek (like I am) you are going to look for anything you can to improve your aerodynamic profile. Aero shaped bottles, besides looking odd to me, pose issues from their irregular shapes - making them hard to grab and hold. Nothing beats a round bottle for ease of use. Putting the bottles behind you and your thighs, right behind your seat, shields them from the wind, making them practically invisible as you move through the air. My first experience with behind the seat bottle mounts was my Cervelo P2. When I sold that bike I had to find an alternative mounting system (since the Cervelo bottle system is proprietary and was included with the bike when I sold it). I decided to go with the Tacx "Seat Clamp" Bottle Mount. I've ridden with this system for a full year now - and here are some honest thoughts on it - PROS: It's robust. I'm a daily rider and I carry two 750ml Camelback bottles with me on each ride. The Tacx Seat Clamp allows you to mount a single cage (in the center) or two cages - Off set from one-another. I have two carbon-fiber cages, the bottles and 1.5 liters of water - all supported by the cage. Getting the bottles in and out of the racks located right behind your seat takes very little getting used to. I can pull my bottles out and put them back without missing - all by feel - which keeps me more balanced. I ride daily as part of my ongoing physical therapy for a serious back injury I took a couple of years ago and NOT having to bend over and grab a bottle from under my seat is a good thing. It's lightweight - doesn't really add too much additional weight to your bike. Use whatever cages you want - I use carbon fiber cages designed to work with the Camelback Podium Chill bottle series. These bottles have a specific shape which "locks" into the cage - preventing the bottle from bouncing out of the cage should I hit a hard bump - THIS IS VERY IMPORTANT! You can set the angle of the bottles in relation to your seat. The more straight up and down they are the less likely they are to fly out. As you will see from the different photos I've attached to this card, I've ridden with the bottles angled in several different positions through the last year. The second photo is the most current representation of the position I prefer to ride. Affordable - I paid $15 for my Tacx Seat Clamp Bottle Mount. CONS: If you are not using bottles and racks which work together to hold the bottles in place, you WILL shoot the bottles out of this rack like rear facing missiles. I took a couple of back up bottles on a ride when I'd been lazy and not washed my Camelbacks - and I have two sets - 4 total - so there's really no excuse - LOL. And about 20 minutes into my ride I hit a bump in the pavement that I hit EVERY time I ride this certain course and both smooth - plain bottles went flying. I cannot see this system working with any success on a mountain bike or trail bike. This rack system really wants smooth pavement to do its job. The clamp mechanism is really poorly designed. You have to really clamp the mechanism down, which bends the mount - to get it to stay stationary on the seat rails under the weight of two bottles. I put a couple of nuts in the tread between the mount bracket (made of metal) and the clamp body - made of a carbon/plastic mix. If you look at the 3rd photo of this card you'll see that even with these nuts and washers between the two parts to keep the tension of the screws from bowing the plastic frame, it is still bowed. Proprietary hardware. BE CAREFUL AND DO NOT LOSE ANY OF THE SCREWS OR NUTS THAT COME WITH THIS ITEM! The bottle racks mount to the clamp using standard bottle rack screws Which means if you don't want to use the screws that fit into your bike frame, you need to buy extras. And VERY IMPORTANT! - these new screws fit into a special square shaped nut - designed to fit into the clamp's body and provide the thread for the bottle screw to attach into. If you loose one of these little odd square nuts, you might as well order a new seat clamp. I put the extra ones I had into a safe place (the first couple of months I rode with a single bottle and rack) and when I decided to move to two racks and bottles I couldn't find that safe place. So I went to Home Depot and Lowes - and nope. That nut is a special sized nut made just for this product. Luckily I did find the safely hidden nuts. Other than finding them I would have had to order a new clamp. YOU CANNOT TURN YOUR BIKE OVER AND SIT IT ON IT'S SEAT AND BARS FOR QUICK UNDER THE BIKE REPAIRS. We've all done it countless times. You turn your bike upside down letting it rest on your seat and bars while you tinker with your bikes crank or derailleurs. You'll not be able to do that anymore. You'll snap your bottle racks off (if made of plastic or carbon - metal racks would probably be OK - but then you'll probably tear your seat clamp off. Really - this isn't the best way to get to the underside of your bike anyway. Some hydraulic disc systems will foul up if turned upside down for any length of time. And with carbon bars and seat posts, you really don't want to put weight on them that they were not designed for. It's a minor thing. But I really felt I should mention it. --------------------------- THE BOTTOM LINE - SEE MY REVISED ASSESSMENT OF THIS PRODUCT - IT BROKE IN HALF DURING A RIDE.... A very convenient way to mount your bottles, saving back fatigue and allowing you to keep better balance (much like if you put your bottles in the pockets on the back of your jersey). Aerodynamically sound. May require some extra attention to detail when mounting. Probably NOT the best solution for mountain biking or any type of riding where you're going to bounce the bike around a lot. The bottles, being located at the end of what can be called an "arm" - not attached to the frame with the frame tubes for support and protection - makes the bottles more prone to launching out of their racks. For under $20 - it is what it is. Not the best designed or best built device but it is an affordable way to try seat mounted bottles - if you decide you like them I recommend you get the Profile Design bottle system - made of aluminum and built to sit below the seat (so you can still turn your bike over for quick repairs) - it's 4 times as expensive - but worth it.