My first lessons were on dry slopes. After that, for the first couple of years, I went along for an hour or so about a month before going away. The last time I went, I paid for an hour, but after 20 minutes, I was bored, so stopped.
So, my thoughts on dry slopes:
Gets your legs feeling real skiing.
You're back in your boots.
You recall how to ride button lifts
After a blustery night on a dry slope (perhaps only applies to Northern Ireland, but imagine today's weather but with more wind), you hit the bar for something to warm you up.
Don't take your own skis, unless you want to kiss your bases goodbye
Don't go within 6 weeks of a ski trip. I know of too many horror stories of people breaking or dislocating their thumbs on the mesh.
Then again, an indoor slope isn't much better, but it's a lot closer to the real thing.
When does the new indoor slope in London Village get finished?