I have the mirena right now and have had it for almost 2 years. it has lightened and shortened my period with no other side effects. My sister was on it for almost 5 years and she got it taken out about 3 months ago and she is 10 weeks pregnant now. it really depends on your body on how it will react.
I've had both the Mirena and the paragaurd (copper) the Mirena had hormones that made me kind of crazy and I did gain a little bit of weight, my periods became a little heavier. the copper one I liked the best since it had no hormones and i didn't gain any weight