It really depends on the type of visa for which you're applying. Medivisas is owned by our own Vicky
who obviously has a load of experience with immigration and visa applications, so if you want to use a professional service, especially if you're making a Tier 1 application or your situation is complicated in some way, then they're a good one to choose!
However, if you look through the threads on this board, you'll find that Vicky (and others) will have given loads of invaluable advice on the various application processes already, so if you're applying for a straightforward spousal visa or work permit entry clearance, hiring representation to make an application for you may not be necessary.