Landis and Leipheimer have been successful with it. Cancellara tried it (see the Tour of California last year), then abandoned it. Now the UCI had changed the rules and nobody will be able to use it. Basically, they now say that forearms must be pretty much parallel to the ground. (That’s a synthesis of what the new rules state.)
Well…yes and no. The rules DO NOT say that the forearms need to be parallel to the ground…and neither does the “clarification” that they issued. The only thing the UCI really clarified last year was that the aero extensions need to leave the base bar “level” (whatever that means)…even though the rules don’t really say that either . All that means is if you want to have your hands above the elbow pads, you need some custom bent extensions that put the attachment to the base bar “horizontal” and then curve up to where you want the hands. If you study the rules, you’ll see that they state that the hands can be anywhere in a box bounded by a line through the steering axis, the top of the seat, the forward extension limit line, and the top of the tire.
As you can see from the TdF photos, Levi didn’t really change his position a whole lot after this “clarification” came down compared to what he was doing earlier in the year. He basically only had to make sure he wasn’t running afoul of the “hand position can’t be above the level of the seat” rule (which I think at the TOC he may actually have been afoul…but not called on it).
Cancellara, on the other hand, was actually faster with the position. IIRC, it was only after being forced to rotate his bars down only minutes before a TT start (Tour de Suisse?) that he basically gave it up. I think the CSC guys just decided to not fight it and went flat to prevent any future potential problems…mostly because the “clarification” was so…umm…clear.
It was actually all pretty silly…
I tried the mantis thing in my own time trialing last year. Had a couple of my worst races of the year with it. It’s nice and low (when your elbows go down like that, your shoulders and head come down, too), but I couldn’t generate power. Can’t say I really gave it much time for adaptation, though. I just didn’t like it. But it’s a moot point now.
Interesting…my experience was completely the opposite. After testing it and finding it to be faster than the same position with arms level, I set some of my fastest times ever with that position. One thing I apparently did different than you, though, was that I actually raised the base bar up slightly to go along with the tipping up of the aero bars. This resulted in my head and shoulders being at the same height as with flat bars and I didn’t accidentally unduly close off my hip angle. I also found that the position is GREAT for being able to power over small rollers since I can use my arms to draw myself forward and just barely touch the tip of the saddle with my arse. This opens up my hip angle and almost feels like I’m running.
Oh yeah…I also did some extended climbing in the position (i.e. ~3 miles at 8% average grade) and when geared low enough that I can keep my rpms in a range of 75-85, I can put out within a handful of watts the same power as if I was climbing the same hill on my road bike. In the extended climbing, with the bike tilted up like that, I also tended to pull myself forward to do this.
So…YMMV 