here - they look great and are a third of the price of other SIMs. We used to have a double air mattress which our three kids piled on to but it was a) huge to pack away and b) it deflated with gusto last summer so was thinking of a thermarest instead (I used to own one and loved it).
I've not used one but watching with interest. I think it looks like a glorified roll mat and my hips wouldn't cope with it - not enough cushioning despite the bits of my own I've been cultivating via quantities of wine and chocolate.
I'm fed up of the air bed and I've been looking at the 7cm double SIM but as you say they aren't cheap
They are good. My DH has used one for years. Definitely at the upper end of the roll mat scale for warmth and comfort.
For your DCs, it depends how old they are, but generally a roll mat is better as they can't deflate or burst it or fall on the floor from a height. A roll mat can also be warmer quicker for them as with an inflatable a small DC's body heat may not be enough to warm up the cold air in the mattress.
We have a 5cm double SIM from Alpkit which seems to be very good so far. The big advantage with Thermarest is the lifetime guarantee on the SI mats - your lifetime - and customer service is good in my experience.
Just to clarify though, my understanding is that they aren't actually SIMs which are made from foam that expands (self inflates even!) so these will be harder than those (only 1.5 cm thick), though also more durable.
If you are interested in an actual SIM, there are many brands cheaper than thermarest. We bought ours from Costco last year and they were brilliant for less than £20 each, 7 cm deep. Unfortunately we tore a huge gash in the one while packing up at our last trip so it will need to be replaced (although, it is actually functional as is, just doesn't pack down and not as springy as the others.)
If you want to go up a grade from them, Vango, Nomad, Gelert, Outwell all do various thicknesses and prices.