Have just spent 3 nights on the Aldi SIMs and have to say they gave the best nights' sleep we've had in the tent - totally converted!
They are very firm and were lovely and warm, the only downside, which quite a few other people have said is that with a sleeping bag they can be a bit slippery if you wriggle a lot in your sleep (youngest son ended up sleeping at the bottom of our feet). So have decided to convert a couple of old duvet covers so I can slip two SIMs in side by side - should do the trick.
Had a look around and I'm rather interested in the Therm-a-rest Dreamtime XLs as two mats can connect together giving 152cms! Very expensive though, cheapest I've seen is £130 each .
We have the Aldi 5cm ones, we've had them well over a year now we left ours open and unrolled behind a sofa in our bedroom they are now inflated to the max lol really comfy!
My daughter has a hi gear 3cm its truly awful i hate it, i don't see any point in using it apart from the insulation value but shes alot lighter than i am and says its fine.
I'm a side sleeper and I use an airbed for normal camping but a SIM when I go off on my bike.
Airbed is definitely comfier. Need a fleece blanket on top or it can be a bit chilly.
The SIM, I just tolerate. It's a thin one to pack up small but I find I get really sore hips and feel bent over like a banana (I'm sure this is worse for us ladies with our sticky-out bits than chaps). It IS good and warm though.
I'm quite heavily built but not huge. I'm sure skinnies find SIMs comfier.
Well I've gone mad and ordered 2 x Therm-a-rest Dreamtime XLs. Using them next week so afterwards I'll post how I get on. All we need now is to get our youngest sleeping through the night instead of waking up 5-6 times and we'll hopefuuly be happy campers! Thanks for all of the replies.