I enjoy using Foursquare only for personal location tracking, I don't use any of the social features, and I've found the data to be quite good. I travel quite a bit (> 100,000 miles this year, 8 countries, more than 15 states, etc.) and the 4sq data has been pretty solid everywhere, surprisingly... I've been able to check-in pretty much everywhere I went except Palestine (understandably!)

One note: Almost all the places I checked into with 4sq were major cities (London, Paris, Gothenburg, Brussels, Rome, Chicago, LA, SF, Washington DC, etc etc.)

Major cities in Poland probably don't really count as major, but both Warsaw and Posen had good data. Same with Amsterdam and Haarlem in the Netherlands, and Fribourg in Switzerland (tiny city, but, on the other hand, a university town, canton capital and all-around awesome, so not exactly some forgotten village).

As a 4sq user in the UK, the data is obviously better in the bigger cities, but the vast majority of restaurants/shops/entertainment places in the smaller towns is present, maybe with just less tips/ratings.

Well the POI data is even worse than Apple's where I am in the UK. Of about 20 shops on the high street, 1 is in the correct location 4 are shown in totally the wrong location but in the town and the rest don't show up at all. On the other hand the map background data foursquare are using is from mapbox.com and its very good; much better than apple has and even better than google with much more local details, like footpaths, that google doesn't have.