This one's pretty good:
A Chinese-English Dictionary of Chinese Idioms by Pan Weigui
ISBN: 7800522377
It's not the most comprehensive with only 2327 idioms and you'd be best getting something different as a reader's idiom dictionary (actually, the ABC dictionary is pretty darn good as a reader's dictionary for chengyu - I don't think I would need a substitute for that), but this one is excellent as a writer's idiom dictionary and certainly has pretty much all the idioms I'm ever going to use in my written work.
I have tried other idiom dictionaries but found them not particularly useful at how to actually USE idioms. The worst will just give one example of usage. Rubbish. This book is better. Examples are given describing when (if) you can use each idiom as a predicate, object, attribute, complement, etc etc and typically giving you example sentences of each type of usage - very very useful as idioms aren't just something you can typically throw into a sentence any which way but loose and expect not to sound awkward. Also, some idioms will typically only be used to modify certain verbs or nouns. Synonyms and antonyms are also mentioned where appropriate. Finally, the bilingual format is always a help; where there is a suitable English equivalent for the idiom, it is given and the English is clear and well-translated while the Chinese, well it's always good to practice your reading. Full Chinese entries are given with English translation only for the idiom's meaning and the grammar (the bits that are usually tough to grasp in a C-C dictionary), but the examples are Chinese only.
To give you a brief example (without breaking any copyright laws I hope!

I'll just give a run-through of what information there is)
A random entry I opened the book on - the idiom shou3 kou3 ru2 ping2
The dictionary gives the Literal Meaning, the Metaphorical meaning and an English Equivalent ("to keep as silent as the grave"). In usage it describes the idiom's use as a predicate, an attributive, an object and a complement and then gives examples of all four usages. As a predicate, the dictionary points out that it "may be preceded by a prepositional phrase indicating the target, for example dui4, or by such words as yao4, bi4xu1 and yi1ding4" (obviously the dictionary shows the characters, not my crappy pinyin). As an attributive, it "modifes nouns referring to people". As an object it "often cooperates with the predicate verb bao3zheng4". The dictionary also points out it can also be used as a complement. The four examples then given seem to have been chosen to illustrate these usages. For example, the attributive example does indeed use the chengyu to modify a person.
I'd recommend that if an idiom dictionary was included in Plecodict's offerings, it should be one aimed at writing, like this one, not just a basic reference, which the ABC does more than adequately.
It's also worthwhile pointing out that most Chinese won't have a clue about a great many idioms in the ABC dictionary, quite a few being sourced from classical documents and as familiar to the average Chinese-speaker as much of Shakespeare or Chaucer is to the average English-speaker. (Hence ABC's usefulness in reading but not writing). A more limited set of a couple of thousand idioms is much more useful for writing essays, browsing for fun and then not sounding too obviously non-native-trying-hard-to-sound-clever when you eventually end up using some of them

.
Other good idiom writing dictionaries?
Ben