Is the Forerunner 255 a good watch ?

For my use it fits the purpose well, and I would buy it again. No issues, but one thing that is a bit annoying. The intended course and the actual course has the same color in map view. This was better in earlier software.

I'm pretty happy with mine. My only gripes seem to be general to the whole Garmin range, not this specific model.

I think the biggest question though would be what kind of screen you want, to decide between FR255 and FR265. I like the transflective FR255 screen and think I'd be very unhappy with the OLED of the FR265, but I realize many folks are the opposite.

Only people with problems come to this forum, so it can seem like there are a lot of flaws with this device.
Personally, I don't have any problems with my unit, apart from the intended course and actual course on the map being the same color, as mentioned by ovekvam. I hope it will be fixed in the next software version, as it was already acknowledged by Garmin.

However if I was to buy that watch again, I probably would go for FR 955 instead. I tend to use navigation way more than I thought I would and without a base map it's sometimes hard to tell which path I'm supposed to take.

My doubt was for the 255. I don't need a tactile screen and for that I read the Oled of the 265 is not so good.

Until now I had no doubt between 255 and 955, and think that the map is not necessary if you have a smartphone which gives a much larger vision. I consider the watch as a help which permit to keep the phone in the pocket. 
But now I have some doubt :((      

With Garmin Pay and navigation built into the watch, I prefer to leave the phone at home if I go running. When going for bike rides, the phone is in the pocket, and it is only used for pictures and emergencies.

Yeah, I'm not a runner but a walker and hiker. I did try an app to show maps on my FR255 (linked to phone) and I deleted it shortly after as a gimmick. My main interest is in the altimeter function and the trip recording. So I might want to glance at timer/distance/elevation stats on my wrist, and not have to stop to dig my phone out for similar data. It also means I don't need to run recording in my phone map app, so I imagine its battery will last longer too on multi-day trips.

But I'm the kind of person who rarely would look at a map while traveling. I have already studied them ahead of time and tend to be well oriented. I expect to have hours of forward route in my head, and would feel panic and disorientation if I didn't know what comes next. I'm the same way when driving too. I never use live navigation but look at routes before ever getting near the car.

A good example where I'd pull out the phone map in the field is when I was snowshoeing and wanted to confirm my belief that the winter route I was following (by trail markers on trees) was converging with familiar summer trails not visible at all under the snow. The landscape was familiar, but different enough in the season to make me want to check my gut.

Best watch I've owned so far. Polar V800 - Polar Vantage V - Coros Pace 2 and now the 255. Finally a watch with accurate (dual band) gps (although the V800 was also very good in that realm). Very rich in specific features for runners and very customizable in comparison with the other brands. Training status and HRV status are in my opinion quite useful for tracking recovery. There are a few minor bugs/issues with the current firmware but nothing serious.

What was the problem with Coros Pace 2?

By the way, I'm also happy with my FR255 watch. I love the MIP display, which I wouldn't change for OLED for any money. For me, that was one of the main reasons I ended up with Garmin.