GPS tech (Garmin, Hammerhead, Wahoo, RWGPS, etc)

I used a 520 to death, so immediately jumped at this gen when it was released earlier this year.

The 540 and 840 are exactly the same, other than a touch screen on the 840. They have the same button layout. So if a touch screen is not something you're interested in, then you can save £100 there. Personally, I'm not sure how intuitive the interface would be for button only, but I'm sure it's perfectly possible.

There's a lot on the x40 that I don't use. The personalized trainer system I don't pay any attention to, live segment notifications, a few other things. But that's ok... no harm them being there.

I've used mapping, and it's perfectly serviceable.... shows the map, gives me directions, tells me if there's a sharp hairpin or busy road up ahead. The climb pro works well, detailing elevation profiles of hill climbs on a pre defined course... it's nice to know what's coming up. The live version of that (when not riding a course) I found a bit hit and miss... sometime it wouldn't give details of a significant hill I'm actually on for some unknown reason. The flexibility of data screens is pretty good, and the IQ store (crowd based apps) has some interesting tweaks one can add (despite it being a shit show for discoverability)

Battery life has been excellent, but I was used to a 520 what lasted for like 3 hours before 0%. I dunno.... 15+hours I guess. I did a 24 hr race round Nurburgring with only minimal topping up.

Can't authoritively comment on the solar version, but I'd hazard that unless you're in a super sunny climate and doing multiple day ultras... it's probably not worth it.

Obviously I've got buyers biased here. I like it well enough. But it is expensive... £350/£450 is a lot for a bike computer. I rationalised the price in my head; as it really is the only one I use and I used the last one for 8 years (well beyond it's life time). If you've other edge's you intend to carry on using for other types of riding... the 540 way overkill. Especially only for commuting or sunday cafe trips. You could easily pickup a 530 for a shade over £200 (although even that's probably also too pricey in these hard times).

And I'm sure other brand of bike computers do similar for less. No idea how polar, wahoo or hammerhead compare feature/price wise; but I did look briefly at the Magene computers as they're super cheap in comparison. Worth investigating at lease.

TLDR: I dunno, I like it... but it's expensive.