Is anyone using the Shimano's Di2 yet or at least ridden it?

Yes to both. Di2 has been around for a while already.

www.google.com

type "DI2 reviews site:www.bikeforums.net" without quotes

I spent a couple of hours test-riding a Specialized S-works Tarmac SL3 that was equipped with Di2. Wasn't terribly impressed, especially given the price...

I have owned the DI2 on my Orbea Opal and rode it for around 6 months.

Here is what I think.

You really have 0 feedback as far as the shifts go. You hit the button the derailleur moves and you don't really feel it.
You can only shift in one gear increments both up and down the cassette.

Is it smooth? Hell yeah.
Is it worth the money? Hell NO.
Is SRAM RED a better more positive shifting experience? YES
DI2 is great for the cool factor.
The one thing that I will definitly give a big thumbs up for is the front derailleur.
Talk about shifting without thought. Hit the button and it doesn't matter what you are doing and it will shift up without skipping a beat.
For me I am sticking with cables for the time being. Having owned it and sold it I can say that I wasn't overly impressed with it. It's just neat.
After riding RED for several months now I don't think I will change to anything else.
Here is the obligatory picture to prove I had it.

It's also EXTREMELY simple to tune.

i wonder if anyone has tried running a Di2 FD only (and a standard, manual, cable rear)??? it seems to me that that would be the ultimate setup especially after reading hodie's review. it would be even better if you could run a smaller battery too.

Yeah, it seems to me that the biggest value is with the auto-trim on the front . . . and the biggest detriment is with the one-gear-at-a-time on the rear. A hybrid system would seem to make a lot of sense.

The problem with a hybrid would be figuring out how to let the front derailleur know where the rear was. They talk through the brain on the bike. It's the little rectangle thing up near the bars.

I think that a hybrid is awesome.

Shimano only makes one battery and they only sell Di2 brifters in pairs. You can buy an entire Red gruppo for less than Di2 front shifting would cost...

i was just talkin hypathetically dude. just saying i think a hybrid setup would be sweet.

didn't know the 2 derailleurs talked to each other. i guess if you engineered this kind of hybrid system you could easily make a sensor that communicated the RD's position to the system .

Both Shimano and SRAM only allow one gear at a time rear up-shifting. The only real reason you need multiple shifts per throw is because the throws are so long to get to a bigger cog. Eliminate the super long throw for down shifting and I bet that suddenly you don't miss that feature.
It seems that the buttons on Di2 can be clicked so fast that multiple throwing on DA-7900 would be slower than just double (or triple, which apparently 7900 can't do) clicking the Di2 levers.

Yeah, or I could buy 6,000 pounds of dirty baby diapers. Why would anyone want to buy a Red gruppo?

Di2 rear shifting isn't exactly fast... at least not on the Specialized Tarmac I test-rode. Probably about the same speed as 7900, if not slower. Certainly no match for Red...

Same with me, 100% although I did not own it, just rode it for a while.

What is this "buy"?

Because I have ridden every top end shifting group there is and RED is simply better.

7900 DA has huge throws. Half of the movement is slop.
Campagnolo Super Record 11 Speed. Don't like how you have to change hand position to downshift although shifting is extremely smooth due to the 11th cog.

That's why they make more than one kind, I guess. I find Red kind of crude, clunky and noisy, although less clunky than Campy. But I'm spoiled by the slick shifting of Shimano. I can't imagine going back to SRAM.

i love the sound of a hybrid system. i would take mine with electric front and my loud ratchety Campagnolo rear that can dump 10 cogs one way and climb 4 the other way. a guy can dream...

I'm installing Di2 on my Cervelo tomorrow (replacing RED). I'll have a full review soon and will be posting install pics shortly.