The UI does suck, but it's an application that I find that I can rely on. Unlike say shotwell (which I've been using for photos) which has a reasonable UI but crashes on me every second time I use it.
On a side note, I find amazing that Goyal falls into this (IMO) idiocy of naming an application one way ("calibre") and then go on telling people to misread name, pronouncing it in some other random way ("cali-ber").