I have a few 2.5 Shellies that I integrated to HA a few years ago. They work great.
But they could work greater!
Question to the ones who flashed ESPHome on them: do you regret your decision?
I have many ESPHome devices so having more homogeneity is better. But this is a bit lying to myself: I just like to try new things. I am writing this post from within a closet so that my wife does not know.
For me, personally, I 100% regret flashing to ESPHome on the Shelly. I did this on a Shelly 1PM UL and got funky power readings and it fell off the network constantly - the non flashed I replaced it with has yet to have a single burp. I still have it in my box of unused automation gear in case I get a wild hair to fool with it, but the Shelly just works as is, I have a couple dozen (not flashed since that one) and have been very happy.
For me, it’s the opposite. I flashed my two Shelly 2.5 with ESPHome, a while back, and have been nothing but satisfied. No glitches or funkiness. I also really like having more freedom to do exactly what I want with the devices.
Example configs here:
PS. I like that you have to hide your tinkering from your wife. WAF is important.
I have mixed feelings. Around 8 devices flashed
1pm
2.5pm
I4
And shutter
2 devices have/had issue of intermittent wifi issues and 1 just died (with noise as end state)
Befote the 2.5 (the older version) had the temperature issues.
Btw there are 3 versions of HW for the 2
5pm.
So overall still satisfied due to the ability to put proxy ble on them…
If I can, I flash everything with ESPHome.
I have done both, but going forward, unless I have a specific use-case where I need ESPHome, I just keep it as-is. Never had a problem and it’s quicker and easier. But I also use all mine for basic things; anytime I have something more niche I pretty much always build something from scratch.
I find the native firmware on my Shelly 2.5 to be perfectly functional. If it ain’t broke, don’t fix it.
Not sure what advantage there would be for esphome other than homogeneity.
That was my intention, and til now I can’t complain. All my “flashed” devices are equipped with ESPHome, all fine here.
Still doing it with every new Shelly device, but I wouldn’t go that far to say it’s necessary. 
I have some devices I flashed and some I left. I am not seeing a pro/con here and that is making me think I will not be flashing them to ESPHome going forward. Maybe if I wanted to do something with them other than the core use, I might. But otherwise all my RGBWs work great, relays, etc… right out of the box. I have a pretty high end network with a MTM / proxy that I can block outbound traffic. My IoT net is 100% off the open internet. I see a TON of traffic going to random places that are hostile to America and Western Values…I have NOT seen that with Shelly OOTB firmware. It is common for me to replace the CPU or otherwise in things I buy - I just yanked the core out of a couple fans and put in an ESP32 I can control - so I am a tad on the extreme of local only.
Personally I think that’s always the best policy with any device. Just because you can doesn’t mean you must. Like @nickrout said, if it ain’t broke, don’t fix it.
Dive into the scripting abilities of Shelly, you can actually do some pretty cool stuff in there.
I flash all my shellies with esphome at first connect and i never had any problems so far. Only device i do have on shelly is BT door/window sensor (it can’t be fllashed to esphome).
Why esphome? You can tinker, add your own commands, make it to work like you want, not like shelly wants.
I have a 1PM using original firmware that I’ve integrated via MQTT as the Shelly integration wasn’t great at the time — it’s been rock-solid for over three years daily switching a 3kW immersion and providing power readings.
I have two 1s switching lights which were super-troublesome on OEM firmware, dropping off the network frequently and not liking the physical switch being operated. I flashed them to ESPHome and they too have been very reliable for three years.
So my answer is no regrets 
Well the summary seems to be
Quite inconclusive, and a matter of personal choice. I’ll add this: Shelly firmware doesn’t annoy you 3 times a month to update the firmware 
Haha… indeed. But, we solved this by disabling that annoying entity, os it’s no panic.
One additional benefit of having esphome on shelly: i connected i2c sensor to one of them to show temperature and humidity in a room.
Then you can have external temperature sensor, import it into esphome-shelly and control heater/climate with it. …etc… endless posibilities …
Would you have it documented somewhere? (what to connect where, and ideally the ESPHome config :))
i have 2.5 in every shutter and honestly i never felt the need to touch the firmware…
you can use them fully local, they do not lack any “killer” feature, so flashing esphome would be a matter of “tech fun”, but in this case i’d go with something fully custom
my 2cents
Sadly, no, i also don’t have that yaml anymore, since i replaced (almost) all my temp/humi sensors with xioami’s, i don’t rember which gpio were used, but perhaps easiest solution is to disable uart and connect to rx and tx on exposed connector - set uart —> baud_rate to 0 and define i2c to these pins. On that connector you also have 3.3v and gnd so you don’t have to solder.
Other option is to follow solder pads and see where are connected to esp and compare with esp chip pinout (or try with beeper). A microscope is recomended for this, though, or at least a magnifying glass.
Me too. But there is this “tech fun” you mention which is a psychological problem for me
But seriously - except of there is a way to upgrade them to do other stuff (per an earlier post) I will probably leave it as it.
i assume this is psychological for a big part of this forum members 
that’s why i’d opt for something different, take a esp8266/esp32, attach a random sensor, find a way to make it work…
the 2.5s are good enough by themselves…
That’s the tech fun, taking nothing and making it something. Taking something and making it something else is also fun. Taking something and making it the same something with maybe an extra bell or whistle seems unnecessary - especially if you dig into Shelly scripting and can do much the same plus you get to learn a new automation system that might have features difficult to reproduce in HA.