Cervelo s3 opinions

Depends how much you like the aero properties. I loved the R3/R5 because of the weight and the classic looks, and loved the speed of the S5. In the end went for a cannondale supersix hi mod which is crazy light and handles like a dream. I'd reccomend trying to take one for a spin if you've decided you fancy a fast race bike  

Not sure where you are based road cycle exchange near Richmond park have a few for sale that you could try out.  I fancy the disc brake version...

https://roadcycleexchange.com/collections/disc/products/cervelo-s3-disc-...

Rode an S5 on a demo day immediately after demoing an R5. It's fast, like properly fast, yet quite stable tho the 65mm rims and wind made the occasional gust 'exciting'. I was also amazed that it didn't shake me to bits and tbf didn't feel much different in the comfort stakes to the skinny seat-stayed R5. It did feel more planted than the R5 though which i found quite twitchy. If you're not massively flexy, then the low front end may not be quite so comfortable as the Roubaix after a few hours, but IME a well fitted race bike is more comfortable than a poorly fitted 'endurance' bike.  I think I'm right in that it'll take 28mm tyres so you can add a huge amount of comfort that way. 

I'll add the caveat that I only had it for an hour of blasting about, but when hopping off it and back onto my 'sportive' bike, I didn't notice much difference in the comfort stakes, only that I was going much slower. 

Never ridden one but like you thought about getting one.  I think they look fantastic!

As well as the seat stays there is a new seat post intended to make the rear end more comfortable. However, they will only take 25mm tyres, so if you don't find it comfortable your options are very limited when it comes to doing something about it.  You will almost certainly find it less comfortable than your Roubaix.  The S3's a race bike and comfort is not its main priority.

Have you looked at the R3 if you want a Cervelo?  It takes wider tyres so can be made more comfortable if you don't get on with it in stock form.  It's probably got a wider range of uses too, being less specialised than the S3.  If you're not set on the brand there are many other options that give similar or better performance for less cash.

I don't have Di2 but I have both mechanical Ultegra and eTap.  The electronic shifting is nice and doesn't miss a shift like you might do if your fingers are cold/tired with mechanical.  Not trimming the front mech is also a boon.  But >90% of the time a well set up and adjusted mechanical system is just as good at moving the chain across the cogs.

S3 wont be great at great distance,compared to roubaix...

Roubaix is designed with long distance and comfort in mind, s3 is much less so.

Then you must also consider your body too, S3 is areo for a reason, if you're above certain weight i.e 75kg you're probably wasting money

Cervelo has all-around racing workhorse that is R3 both comfortable and very much areo, as well as C3, more gravel adventure bike

unless you're getting into triathlons and TT getting any areo bike won't give you flexibility for different roads and weather conditions.

I've recently had rental bike with Di2 Ultegra on it...

I could never justify buying one on my main bike, and to tell the truth i prefer mechanical gearing due to feel and feedback received on 80% of road situations.

I only found it useful on long climbs when my legs are giving up and i need quick gear shift, mechanical will have a longer throw to engage largest sprocket, with DI2 is a simple click, same goes for front shift, when engaging large chainring, simple click without palm gymanstics

After years of cycling with mechanical gruppo i am able to prepare gears before i need them, so di2 is more of a fun gimmick that i won't be investing in anytime soon...

A bit like division between manual and automatic gearboxes really... I live in a hilly area of london, that gets traffic 360 days a year, driving manual was driving me mad, so i bought automatic car simply for convience sake

Just a note; isn't the S3 supposed to be an S5 front end but with a slightly less aero but more comfortable rear triangle?

What are you riding now ?

So you pre-order one and don't get on with it... What happens then?