How effective are guided meditation sessions in achieving the benefits of meditation? I find it difficult to meditate without guidance. I wish to use guided meditations for my practice.

How effective are guided meditation sessions in achieving the benefits of meditation?
What is the best way to use guided meditations to maximise their benefits?
For the purposes of relaxation, they are very effective.  If your goal is simply to de-stress, then by all means use what works.  Even the Buddha taught a gradual path and tailored his teaching to the station and goals of his audience.  Guided meditation is one shade on a spectrum ranging all the way to enlightenment.

However.  While guided meditation is an excellent way of slowing down mental chatter and decompressing, that's about as far as it will take you.  If you are really interested in Buddhism and wish to meditate for the purposes of developing insight and penetrating into the four noble truths, you have to go past the relaxed state of mind engendered by guided meditation.  You need to develop concentration.

Concentration can only be developed by you.  It is a choice that you make every moment - a choice to return to the breath when your mind starts to wander.  It's no different than physical exercise.  The more often you make the choice to maintain your concentration, the stronger your meditation becomes.  It also gets much easier with time.  Concentration is also subtle.  While you can use props like a candle or another person's voice, objects like the breath, kasinas, the body, etc. require much more focus and delicate handling.  Too much effort and they dissolve.  If you don't apply enough, they slip away.

I hope this doesn't come across as me saying that there is anything necessarily wrong with meditating simply to calm your mind.  Destressing certainly has value.  Just be sure to ask yourself what your goals are.  Your answer will determine how to make the most of your time on the cushion.