It depends on the needs of the learner. Prefix or Suffix can at best give you an idea of the meaning in the broader sense. I can take a wild guess, say for example, the suffix, mania means madness, cide which means killing, the prefix, mono meaning single, the prefix bi, meaning two and so on.

Down here, I find students using this approach mainly for answering multiple choice questions, in various competitive exams, where time is a critical factor.

A student or professional who is focussed on improving his vocabulary, can in no way take the easy way out. I always ask students to make it a habit to read any newspaper or magazine related to their area of interest. Unarguably they are bound to encounter lot of new words. They should first try to guess the meaning of the word with respect to the whole sentence, after which they can look up the dictionary and understand the precise meaning, part of speech and pronunciation. Once they understand the meaning, it is imperative for the learner, to form a sentence using that word and get it corrected. Other methods never work. The use of the right synonym is also critical to convey the message in the way you want the reader to understand. These subtle nuances of the English Language can be understood only when a learner strictly adheres to the aforesaid process.