I have 2 Di2 bikes, one of which I've owned for 5.5 years. The second one I bought not for the Di2, but because I got it for the same price as mechanical in a sale last Spring. I think you can forget finding any bargains from now on because of the covid/brexit combo.
Everyone's values differ, but for me the real value of Di2 is in winter. I really think I wouldn't bother for a summer bike. In winter cabled gearing deteriorates pretty quickly to the point that when you eventually change inner and outer cables it feels like a brand new bike. This doesn't happen with Di2, and the truth is that once you have set it up it is maintenance free (apart from charging of course). Equally, if you have cold, tired hands on a long winter ride Di2 is magic. No more clumsy lever throws and crashed gears at the end of a 200k in January. It shifts smoothly every time. If you are crap at changing gear anyway you will still be crap with Di2 (ie. low cadence changes etc) but the really noticeable difference is it is way simpler to change gear when out of the saddle going uphill. Some people claim that it is dificult to shift in winter gloves but I haven't found that to be a problem.
The downsides: Yes, you've got to remember to charge it. Never been a problem for me. I have a plug with a usb cable right next to my bikes for charging the plethora of lights, headphones, gps devices and di2. Contrary to what somebody says upthread there is a light indicator on the charger. There is also a charge indicator on the junction box activated with one press on any lever. You also have to remember to pump up your tyres (far more frequently) but nobody moans about that.
If it does go wrong midride you will be stuck in one gear, and it might not be a gear of your choice. However the number of times this has happened to me is zero, whereas I've had cables snap every few years. Yes, you can carry a spare cable, but it is still a ball ache. If Di2 does stop functioning it is much harder to diagnose and fix both as an individual and for a shop. However, in general, it is a very robust system. From what I can tell most issues arise from people pinching cables in BB area or seatpost area.
Expense: yes, it costs more than a cabled system. Is it worth it? Well, it isn't a clear yes. Law of diminishing returns comes into play, and it comes down to personal judgement. My view is that it isn't necessary, but it is a pleasant luxury. I'd avoid Dura-Ace (despite owning one) for this reason. Replacement Ultegra rear mech can be found for £170. Replacement Dura-ace about £450. And the functionality is pretty much the same. The pricing is not dissimilar to high end Campag, in that levers are simple and cheapish and the rear mechs are costly.
Compatibility: I don't think this is a system you buy intending for it to last 25 years. Although it may well do. I think you'll find the same issues with geared systems. 25 years ago we were on 9 speed. Can you still get a 9 speed Dura-Ace mech? It will be interesting to see what happens with backwards compatibility.
TLDR: If it was for a summer only bike I wouldn't bother unless you are loaded. If it was for a winter bike then yes, as long as you are aware of the downsides.
It's a precaution. 🤔
