Casual. Two flavors: genuine admiration ('hats off to you'), and exasperated surrender ('I can't deal with you'). Tone and context distinguish them. Commonly written 我服了 or 服了你了.
memory
服 literally means 'to submit / to yield' — same 服 as in 佩服 (admire) and 说服 (persuade). 服了 = 'I yield'.
