I had an 250 watt Kitchen Aid for about ten years and it gave good
service though it seemed a bit underpowered for kneading larger
batches of dough. For Christmas, my wife gave me a 325 watt model
('Artisan') and it seemed to strain even more when kneading even a
six cup flour recipe. So I've had to upgrade to the 'Professional'
level of Kitchen Aid. Confirming this about two years ago, a cooking
magazine (maybe Cooking Light) evaluated about 15 stand mixers and
rated only the top level Kitchen Aid and a Hobart as 4 stars for kneading.