I lived there for a year in 2004/2005.
Chabad is a nice frum place – very decent people.
Recommendations: personally I didn’t like the ‘old’, eastern side of the city center; I preferred the so-called ‘new’ side on the western side of the river, since it is much quieter, more authentic. My biggest recommendation is to walk all the way up to the castle at night from the eastern side, exit from the western side, and walk back down from there. You start walking uphill near Malostranska metro station, and back via Malostranske Namesti and the old bridge.
Personally I think Prague is much more beautiful in winter, with snow, than in summer. But it’s a wonderful city anytime of the year.
Just be aware that there are quite a lot of pickpockets and quite a lot of anti-Semites around. (Statistically maybe 1/10000, but meeting one can be enough to seriously wreck your day.)
Kashrus is difficult – that is, there *is* kosher food, it’s not really a problem, but the kosher bread is not very amazing (ehm, ehm) and getting things like cheese can be challenging. But I do think it’s easier nowadays than when I was there – for example, Chabad opened a small simple restaurant now; that didn’t exist back then.
In any case, it’s a great place to visit and I myself hope to go there some time soon with my wife as well.