I bought myself a gravel bike at the tail end of 2016 for £355 in the end of season sale. Primarily I bought it simply as a winter hack so I could keep my road bike in 'summer spec'. A Raleigh Mustang with a pretty basic spec - Shimano Claris, Tektro Mira mechanical disc brakes etc.
In all honesty... I love it, I probably prefer it to my road bike for the vast majority of my riding. It spends most of its time on the road but I want to keep the option of a bridleway available at all times so I now run 38 mm G-One All rounds on it. Over many thousands of miles I can confirm (via Veloviewer) that the average speed on it is just 0.6 mph less than my road bike
It does feel more slugish than the road bike but the speed is closer than I'd ever have imagined. It would be interesting to swap in a set of 28mm slicks like I run on the road bike and a better set of wheels to see how much closer that 0.6 mph gets. The off road miles have obviously eaten into that average over the years too...
I've done three "Roubaix" events on it, 75 miles with around 15 of those being off road which it just soaks up. I've also (Covid permitting
) got a 65 mile 50:50 on/off road event planned for November. What I really like most about it though is the ability to just map out a random route and ride it. Accidentally incorporate an off road section? Oh well! Last time I tried a random route on my road bike I came across a patch of deep gravel and binned it!
I'd certainly recommend one as the stereotypical "one bike to do it all" scenario. Obviously I'm a hypocrite though and have convinced my wife that the correct answer is in fact 4 bikes being the minimum number I need