I'm a teacher and a professional translator. I've been using Ultralingua through all its versions since around 1999 when it was one of the first electronic dicos for Mac on the market. Over the years I've experimented with, but definately not bought, many others on both Mac & PC. I use Ultralingua for Spanish/English, German/English, and German/French, and the monoligual French dictionary and thesaurus.
Pros: Simple download and ease of installation. Wide lexical base for collegiate and non specialised pro use. Speed: results as-you-type. OneClick back translations. Lots of examples of language in use. The grammar sections, the conjugator, the embedded forum, and best of all the HOTKEY (which launches the app automatically) are all additional features that don't come with most other dictionaries in the same price range. The flashcard feature is useful in teaching situations.
Cons: Minor: Needs some improvement to the language selector system when working in multiple languages on one document. Not enough free upgrades to the lexical databases. Could do with an 'insert' button to put a found result directly into a work document.
Summary: I've always appreciated its uncluttered resizable interface and results display. The lexical info is excellent and offers more than most dicos in the same price range, including IPA pronunciation, irregular German plurals, pre and post spelling reform, and regional variations. There may be an occasional word missing, but it is not a intended to be a complete lexis for every language in the range of products, and does not mar its quality. Other dictionaries from large traditional publishers have a similar rate of inaccuracies. Particularly useful is the conjugator although I don' t personally use it much except for rare subjunctives etc. Very stable application, rarely if ever, causes any crashes. Low memory and CPU consumption, and can be left runnig in the background. I think the bad reviews are spam from competitors or people using old versions. I've always had good, friendly service support and a refund for anything that didn't work. Those who don't get replies from customer service probably don't express themselves politely enough. I'm very satisfied, I buy the pay-for upgrades willingly, and I see no need to change.