Hi there.
I did one drop-in group session a fortnight ago and it was OK, just not a very relaxing environment. My proper course starts tomorrow, 2 hours every Saturday.
I've been following the Mark Williams ("Finding Peace...") book for about five weeks now. I'm also working my way through "The Mindful Way Through Depression" by Mark Williams, John Teasdale, Zindel Segal & Jon Kabat-Zinn. Also Jon Kabat-Zinn's Wherever You Go, There You Are. All excellent and highly recommended.
The Mark Williams book is designed to ease you in gently. The meditations do get longer and deeper. Most days I do two sessions of 30 minutes each. Today I've done nearly three hours (including some light yoga practices) as I really felt that I wanted to.
I'm still finding it enormously beneficial.
I like to vary the audio tapes to keep things fresh. There's quite a few good ones available online at the usual pirate sites. Jon Kabat-Zinn is my favourite instructor. He also happens to be the leading light in Mindfulness worldwide.
If my course goes well, I'm thinking about spending some time on a retreat later this year, when I can afford it.
It's really working for me. Before I started looking into Mindfulness I was in the midst of awful depression. I still get my dark moments but even those are now manageable.
I wish you all good things, Jon