I would not say med entry is necessary but I would recommend it. However it's only any good if you know how to use their endless archives of questions and the practice exams. Section 3 is really what medentry is good for - do a few trial exams and find out if you are good/weak at this section and let that help you decide whether or not you need medentry.
Medentry's section one might help - but in all truth it is completely ridiculous as the drills and the questions are so hard that they offer little relevance to the real UMAT (you will often find that you may need to spend 40 minutes to over an hour on a single 20 minute drill) - that being said, section 1 in the exams is decent.
Section 2 is pitiful IMO, for me, the more of it I did, the worse I seemed to be getting.
For section 3, one of the best ways to improve is to just do a lot of their questions (at reasonable time intervals, don't go overboard and do too many at once) and that should noticeably improve your scores over time. However a lot of people still seem to claim that mass-doing medentry s3 questions still does not seem to help.
Most of my colleagues at monash med seem to have some experience with UMAT tutoring - and there are quite a few here who have done medentry (although they have varying opinions on how useful the course actually is)
Last but not least, doing med entry/any UMAT course will always give you invaluable confidence and some idea of what the UMAT is like - and even if the questions are poor in some cases, at the very least you can use them to practice your timing and speed making medentry a very worthvile course IMO.