Not really stiff and thick enough for a normal mid-thickness pile carpet. Might be OK on berber or really short pile. If you have a mid to thick pile carpet get the thicker one. Better than the thinner ones but still not enough.
I thought it would be the best for a part floor part carpet situation but it moved every time I used it as it was primarily on a carpet. I bought the carpet model and it works perfectly as the floor part is minimal in my situation.