Which dictionaries about idioms, collocations, proverbs, word combinations, etc. do you use, how good do you find it?
To be honest, Lana, my idiom/phrasal verb/etc dictionaries are gathering dust upstairs, because I have found the internet to be so much more convenient. The trouble with those dictionaries is that each one only shows a part of the seemingly infinite list of such word groups, and I often have to look through several before I can find what I'm looking for-- if then.

Between Google, OneLook Dictionary and a web concordancer, I have about everything I need.
web concordancer?

Lots of my fellow students used to have a little booklet called the BBI collocations or something like that. I never bought it, but recently I had to translate some phrases and I couldn't find all of them, so I started thinking that it might be a good idea to buy something like that. I just thought I'd ask here, before I buy something completely useless.
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Yes, I have that-- The BBI Dictionary of English Word Combinations. It is quite useful for common collocations, especially for verb + prep/adv.
See. I'd rather have something that covered everything, but I guess it would be almost impossible to find such a dictionary? I do have problems with prepositions sometimes, so maybe the BBI wouldn't be such a bad choice.

I find the Longman Advanced Learners Dictionary has quite a huge collection of idioms/collocation and spoken phrases.
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The BBI is my Favourite (capital F). It doesn't list all the prepositions but it is extremely useful for ESL speakers because it tells you how to combine nouns with adjectives and how verbs are used (it gives 19 valency categories of verbs). It also gives the most common word combinations, as well as some idioms and phrasal verbs. The dictionary also points out some differences in usage among American English, British English and Canadian English. I strongly recommend it!