I have used the twin flex two years.  It has gotten slower to heat up.  It also steams a lot when making sheet pan dinners and the upper oven rack
 is too close to the element so over browns chicken and must watch casseroles.  I must use lower oven for 9x13 casseroles with slightly tented foil covering on them. Cookies seem to do OK.  I bought a Oster Roaster for Thanksgiving turkeys.