The Slate is an odd bike: it doesn't really fit into a category, but I don't think that matters. Riding a bike should be fun, and it's fun to ride on a lot of surfaces: on-road it's fast and smooth, off-road it can handle a surprising amount of abuse. If anything, it fits in the category of fun, versatile bike.
I've had my Cannoldate Slate Ultegra since 2016 (size large), and have ridden 3,088km on it, according to Strava. Along the way, I've changed a few components which I'll list below.
Tyres: Schwalbe G-One Allround 1.35
I've gone through a variety of tyres, having finally settled on the Schwalbe G-One. Please note, it's not 'shwal-bee', but 'Schvalber'. Oder? When the Slate came out, there weren't that many tyres I could find to fit it: I rode on the original slicks for about 1000km, then tried some Surly Knards which changed the handling a bit too much for my liking, and got far, far too many punctures, despite being tubeless.
I find the G-one provides good grip on the baked/loose dirt that the Bay Area has a lot of, and I don't have an issue with how they roll along the road. Tubeless, with the new Finish Line liquid, is the way to go.
Handlebars: Specialized S-Works Aerofly Carbon
I found the original stem a bit long, so for a while I rode with a Truvativ Holzfeller Stem 40mm 0deg. I then found out about riser drop bars, and then the S-Works Aerofly carbon. With the 25mm of rise, I was able to go back to the original stem, and can comfortably spend 2h+ in the drops without my back complaining.
It helps that they look impossibly cool, especially with bright orange bar tape, that highlights the flat black aero tops. And that they're aero.
Seatpost: Ergon CF3 Pro Carbon Setback
Inspired by Jeremy Brown's 2000km review, I upgraded my seatpost to the Ergon CF3, which made my Slate the most comfortable bike in the seated position since I last rode XC full suspension.
The roads in the Bay Area are mostly terrible, and this seatpost lets me stay seated, even when riding over deep potholes or bridges made out of old railway sleepers. Coupled with the Lefty Oliver, it has a lot of the feeling of full-suspension comfort, without any of the sag. Riding my Salsa Fargo, with its 3" tyres and standard seatpost, is a lot less comfortable.
Drivetrain: SRAM eTap RED
This upgrade was originally to going to be to Di2, but the appeal of wireless got me: my Slate is now a Cannodale Slate SRAM eTap RED, not an Ultegra. At the time time as switching the shifters (and brakes, et al), I switched to SRAM's 11–32 cassette. This in turn made the climbs that had been a bit of a chore with the original 11–28 range much easier, and is a change that I feel should be made at Cannondale's end.
A word of caution when switching to eTap: the front derailleur will limit your tyre choices even further, as the battery sticks out into space that would otherwise be occupied by a rear tyre. I originally tried the G-One 1.2, but it changed the feel of the bike too much for my liking, so I found that Schwalbe also make the G-One Allround 1.35. This fits with about… 2mm - maybe 3mm - of clearance, and I have to confess concern about the tyre touching the battery on rougher terrain. The rest of the time, the ride and shifting is so sharp that I don't care.
Frame bag: Apidura Mountain (Large)
For longer rides, I add the Apidura Mountain frame bag, which lets me put a 3L water bladder, snacks, and something warmer securely in line with the frame. It leaves space for a water bottle too, so one could conceivably carry a 1- or 2L bladder in the bag, and electrolytes in a water bottle, leaving more sandwich space.
I dabbled with larger under-the-seat bags, but really didn't like the sway the induced.
The original components are the cranks, chainrings, wheels and stem. If I was to change anything, it would be to carbon cranks, and a modular SRAM chainring setup, allowing me to swap the rings for a specific ride. That said, the cranks are stiff, and the whole bike is so light anyway, that I really don't think it would be worth it.
Notes on the suspension
I haven't had it serviced yet, and nor does it feel like it needs servicing: it steel feels as responsive as when I first rode it.