I have visited the Euler Institute two times, in 1992 (Lobatchewski conference) and in 1997 (geometry conference in honor of A.D. Alexandroff).
The scientific content of both conferences was of a very high level. Top specialists in geometry were present at the conferences, from Russia and from abroad, and the content of the talks was of the highest level. The atmosphere at the institute, between the lectures, is very favourable for mathematical dicussions.
Both stays were very pleasant. There is a lot to see in St Petersburg and in neighboring cities (museums, monuments, concerts, opera, history). The staff (computer scientist, secretaries) is very nice and very helpful, for matters regarding the functioning of the institute itself, for practical matters such as housing, meals, etc. as well as for advices concerning train tickets, theatre tickets, etc.
For both conferences, I was impressed by the quality of the organisation.