Theres no problem with U processors.... they are slower and consume lesser amount of power compared to M processors making them suitable for ultrabooks.
It is not that is it bad or good it is just that what games are you going to play with it. For sure u series will not game on high comfort level that a m processor can do. They will be able to game but will heat up rapidly due to lack of cooling vents. But yes the battery will be good. And along that they will be sleek and stylish, but my word of advice if you game just be away from these processors.
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