A monument of philology, but a couple of caveats, July 11, 2003

Uriel Weinreich was one of the top scholars of the Yiddish language, and to a lesser degree, of Yiddishkeit. He was nonetheless overshadowed by his father, the great Max Weinreich, who wrote a four volume history of the language (in Yiddish, with a one-volume English abridgement) and who started the massive Yiddish-Yiddish dictionary (under the aegis of Columbia University, I think) which is still "under construction".
There aren't many good Yiddish-English dictionaries out there. Uriel Weinreich's is one of the two I'm familiar with, the other being Alexander Harkavy's Yiddish-English-Hebrew Dictionary.
Weinreich gives good grammatical information in his entries, such as verb aspect and case of verb object, along with unpredictable forms such as the past participle. Both alphabets are very clear and distinct, and big enough to be readable for those past the bloom of youth. The English-Yiddish section is valuable for those using the book in Yiddish classes and for those who would like to speak, as well as read, Yiddish. Of course, it decreases the overall size of the dictionary, so a lot of the words you encounter won't show up in it.
The big drawback is the work's prescriptive nature, meaning that this is how Weinreich thought Yiddish should be spoken and written, not how it was spoken and written. Critics such as Solomon Birnbaum have even claimed that Weinreich made words up, if he didn't find them ready to hand. True, new words are formed or borrowed all the time, but that's the job of writers, subject specialists, members of subcultures, teenagers, and grannies, not lexicographers.
Further, if your goal is to read classic Yiddish literature (Perets, Sholom Aleichem, Yitskhak Manger etc.), this is not the book for you. I would recommend Harkavy's dictionary, if you can find it. He doesn't give the noun genders, there is little grammatical guidance and the print is hard to read, but the word you're looking for is likely to be there. And the book's very age (1920? or somewhere along there) is a plus, for this purpose. Of course, any serious student of Yiddish needs to keep good Polish, Hebrew and other dictionaries on hand, otherwise many words will remain a mystery, whatever Yiddish dictionary is used.

Excellent Yiddish Dictionary, December 3, 2001
Reviewer: A reader
The main reason I purchased this dictionary is because it actually retains the original Yiddish orthography, that is, the Hebrew alphabet. Many other dictionaries I saw were transliterated.
Whenever I want/need to look up a Yiddish word, I find this dictionary has it. It is extremely thorough and up to date to the things that need to be expressed today. Many times, if I look up the major word in an English idiom, it lists the equivalent Yiddish idiom, which is very helpful for writing in Yiddish.

One of the best, if not the very best on the market!, August 13, 2001

Who ever wants to learn Yiddish, to write it, to read it, needs to have it! Teachers should have it "at their bed-sides"! I do recommend it as a must have! FRIDA (Grapa de) CIELAK Author of the Yiddish set of ARELE Workbooks. Founder of the International Yiddisher Briv-Fraynd Club (Pen-Pals, in Yiddish) Website: [....]

More comments on this matter, September 14, 1999
Reviewer: A reader
As a student at Binghamton University (SUNY), I would like to state that Scott Eckers is a tool. The dictionary is very informative and has every expression imaginable in it in both English and Yiddish! Unfortunately, Mr. Weinreich lists many Yiddish words for each English word, without sufficient commentary on each word or phrase. It is sometimes hard to decide which word to use in the context of my sentence from the large list he gives. All in all, I would recommend this dictionary - it is the most complete English-Yiddish Yiddish-English dictionary I have seen. I do feel, however, that the more advanced Yiddish student or speaker could benefit more from this book than a beginner.

Dictionary is complete but complicated, April 29, 1999
Reviewer: A reader
As a Yiddish student at Binghamton University (SUNY), I purchased a copy of Mr. Wenreich's dictionary to supplement my textbook and to learn more about the Yiddish language. It is very informative and has every expression imaginable in it in both English and Yiddish! Unfortunately, Mr. Weinreich lists many Yiddish words for each English word, without sufficient commentary on each word or phrase. It is sometimes hard to decide which word to use in the context of my sentence from the large list he gives. All in all, I would recommend this dictionary - it is the most complete English-Yiddish Yiddish-English dictionary I have seen. I do feel, however, that the more advanced Yiddish student or speaker could benefit more from this book than a beginner.