I am about to purchase a new cervelo p2c and can get it with DA 7800 or 7900. What are people hearing or experiencing with the new vs the old??? The price difference is a little over $200.
Thanks!
for only $200 - get the 7900, if only for the new Brifters as they are quite a bit nicer (adjustable reach, internal cable routing, etc.)
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Some of the reviews I am reading are that the pros are finding them unreliable and prone to failure.
LOL! Guess you better just “make the leap” then and go all SRAM Red. All the cool kids are doing it! 
for only $200 - get the 7900, if only for the new Brifters as they are quite a bit nicer (adjustable reach, internal cable routing, etc.)
He is buying a P2C… I think it would be safe to say hes not concerned with Brifters…
7900 - the only reason i say so is that it might be easier to get replacement parts 3 to 4 years down the road.
7800 is great stuff, though, so it’s not for performance reasons that i’d go with 7900 instead of 7800.
Doo two hiz lack of edjucation RRoof just kan not speel and iz terrable wif bike nowledge and his Brifters R two hi.
That being said, even with bar ends the 7900 will be more the norm as we go forward - for the $200 why not get the newest of the new?
I haven’t looked at the latest full spec, so forgive me if I assume incorrectly, but based on the spec when I got my P2C, they used FSA cranks, house brand (Tektro?) brakes, and Ultegra-level cassette/chain, so about all I would have gotten for the DA upgrade (over their Ultegra build kit) is derailleurs and a better training wheelset. So in this case, 7800 vs 7900, is there any diff in the wheel spec or the chain/cassette? Otherwise, you’re basically just paying $200 more for the derailleurs. Not worth it. If you really want the 7900 derailleurs, you could probably just buy new OEM take-offs on eBay for about $200, sell off the 7800s, and come out ahead enough to buy a new set of tires, or an aero helmet, or a couple gift bottles of top-shelf Scotch, etc…
Interesting that you mention the issues with the 7900… I read in Cycling Weekly that for the Tour De France, the Cervelo Test Team went with “box fresh Shimano 7800 groupsets fitted with one alteration: the cranks”.
They show Sastre’s Cervelo R3SL up close in the July 16, 2009 issue. Cranks were Rotor 3Ds (special edition Tour yellow and black to match the frame).
the new 11-28 DA cassette is only avail for the 7900 if your into that sort of thing.
This was all very helpful. I think I’m going to go for the 7800’s. No one seems to have a story of the action of the 7900’s being a lot better. (I read that you don’t have to trim the front on the 7900’s. Not sure that is worth it.)
Interesting that you mention the issues with the 7900… I read in Cycling Weekly that for the Tour De France, the Cervelo Test Team went with “box fresh Shimano 7800 groupsets fitted with one alteration: the cranks”.
They show Sastre’s Cervelo R3SL up close in the July 16, 2009 issue. Cranks were Rotor 3Ds (special edition Tour yellow and black to match the frame).
I believe I read somewhere that Cervelo had to go with the DA 7800 because the 7900 front drivetrain would not work with the Rotor 3D’s. Cervelo HAS TO USE THE Rotors because they are major sponsors of the Cervelo Test Team. If they were allowed to use the DA 7900 crankset, then they would have used the full DA7900 group.
I have the DA7900 full groupset on my RS. NO problems after 3000 miles in dry and wet weather.
So, why not use the rest of the 7900 groupo – i.e. brakes, levers, etc… and the 7800 drivetrain? We know cost is no object with these bikes…
I am looking at the Shimano Interchangeability Chart and the DA ST-7900 Levers are not backward compatible with with the DA7800 front/rear derailleurs. Which means that if you have to use the Rotor Crankset, and the DA7900 front derailleur won’t work, but the DA7800 front derailleur will work. Then you cannot use the DA ST-7900 Levers, which means you also cannot use the 7900 rear derailleurs.
The DA 7800 brakeset is compatible with the DA ST-7900 levers, so they could have used it on the Cervelo Test Team bikes… but it wouldn’t be a color match with the rest of the DA-7800 group 
.02 cents
Not worth the mismatch…
It’s too bad everything is not back compatible…
I’m sure if there was a performance difference, Cervelo would have used something other than the 7800.
Some of the reviews I am reading are that the pros are finding them unreliable and prone to failure.
really? I haven’t really seen any of it failing, and locally that’s what most guys are using.
I have about 5,000 miles on mine, a lot of it in lousy weather. I don’t think I’ve made a single derailleur adjustment since I installed it. For me, it’s actually required much less frequent attention than the 7800. That said, I’m really not thrilled with the shape/size of the brifters.
I am looking at the Shimano Interchangeability Chart and the DA ST-7900 Levers are not backward compatible with with the DA7800 front/rear derailleurs. Which means that if you have to use the Rotor Crankset, and the DA7900 front derailleur won’t work, but the DA7800 front derailleur will work. Then you cannot use the DA ST-7900 Levers, which means you also cannot use the 7900 rear derailleurs.
The DA 7800 brakeset is compatible with the DA ST-7900 levers, so they could have used it on the Cervelo Test Team bikes… but it wouldn’t be a color match with the rest of the DA-7800 group
.02 cents
you’re looking at an old chart…
The only thing that’s not back-compatible are the shifters and the FD. Cable pull/parallelogram geometry is identical on 8/9/10 speed, with the exception of 8 spd DA.
I’ve been using a 7703 (triple 9 speed) rear derailleur w/7900 everything else, flawless shifting.
Do you find that the reach seems too far out on them? Mine feel a bit too outbound and I understand they are not adjustable like SRAM, right?
The only thing that’s not back-compatible are the shifters and the FD.
And I assume that is irrelevant with a left, friction bar-end, shifter.
Victor
Just installed DA 7800 on my new bike. Saved a ton( on the road group). It’s working fine.