I did it last year. Lähti is pretty close to Helsinki so good for sights etc before or after.
The town is nice enough with nice cafes.
The swim will be wetsuit and is self seeded wave start.
The bike course has more hills than I expected. 2 hills were a real grind in in my 39-25.
The run is flat and along the shore for a while.
The weird part was the afternoon start. As its close to midsummer the sun sets at around 11pm.

"Lahti" :)
Hopefully they add a few buoys for the way back, as some ppl veered off-course and ended up crawling over a shallow reef.
The posted course profile showed a few 6%-8% climbs. For a closed bike course, there were surprisingly many small cheering sections along the route.

Is US races, you generally receive an event shirt at packet pick-up. At Lahti, picking up the shirt was part of the post-race activities, separate from getting your medal at finish line.

Easy enough travel wise. I found the area a bit more industrial than Sweden where we went after by ferry to sightsee. Plenty of affordable places to stay. Not fancy at all. Lahti is like a small FInland city and one that has a 3 block downtown. FOr the most part, the majority of athletes spoke English but some of the locals that weren’t racing got downright trashed and were a bit brutish. I almost had to fight one off my prude teenage daughter. Think maybe along the lines of Eagleman (Cambridge, MD.) as a comparable size wise. Expo was nothing to write home about.

The weather this year was complete shite. Cold, rainy and windy. Be prepared for all possibilities and wind.

Swim was bit rough but nothing too bad, as it was a probably more of the weather. Wetsuit for sure. The bike as said above is way more hilly than we expected. Beautiful but slow. The run is crowded and out and backs. Flattish and good crowd support.

You are close to loads including Russia, Sweden and Norway, and Northern Central Europe. All in all was decent. I think I would prefer to race in Sweden given a chance. That country way more impressed me.

I live in finland, so might be bit biased :)

Weather:
Last summer was propably warmest and sunniest from may to september. However that particular race day was cold and rainy. In practice you can expect anything between 60-90 fahrenheits. Swim will be wetsuit. The likelihood of non wetsuit race is 5% or less.

Swim:
As earlier said, some more markers on the swim leg would have been good. However the race course was simple and the last couple hundred yards is pretty cool as it goes past the crowd to the marina.

In general (related to the markers) Pretty sure the organizers will look into those small updates. I know personally some of the lead guys in there and they are professionals when it comes race organisational things. They have build most succesfull triathlon series in finland over the past years. As this was the first race on this site with the course, these small things will get updated.

Roads/ride
Few roundabouts at the sart. Its mix of some rolling hills, but quite fast and flattish. Depends also on the wind angle on how thouh the race is. He road is closed to traffic. And finnish people were here and there on the course cheering the riders. Roads were patched a bit prior the race last year, so there was nice smooth aspalth here and there.

Run
Run is few rounds next to the lake, lots of people cheering especially when passing the race center. Heard that they might do some changes on this place to make it easier for crowd to go from place to place.

After the race
The was proper meal after the race. However the showers were pretty far.
As said earlier the race start is late afternoon, so last ones come to finish in mid night. Well sun is up anyway, but it can be bit cold.. depending of the weather of course.

As a comparison i did Ironman ST george 70.3 in 4.38 and Ironman Finland 70.3 4.23..( in finland i was in better shape, but still loaded with ironman preparation for tallinn in early august)

Other
Last year the race was live on national tv as well as online.
Lahti is know as race city. There has been few world champs in skiing as well worldcup race each spring. Its pretty easy to get by car, from airport to via highway its about 1h15min. If you are first time in finland i would say to check helsinki. If you are longer then perhaps to to visit tallinn as well (couple hours via ferry). Even longer then perhaps visit lapland.. you can also get to ferry and visit stockholm via overnight boat..

More than happy to answer any questions.

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Finland is the only Nordic country which I have not yet visited so this race is definitely on my bucket list. But I've got 4 years of trips stacked up in front of it, so I hope the race lasts a long time!