Lounge is woefully small at St Pancras.... Even smaller then the one at Waterloo.
Paris lounge is much better with priority boarding

Definately not worth the extra though, but if you've got AmexPlat you can get into the lounge gratis. Also helps you at check in at Paris where the queues are bad due to them letting people through only near the departure due to the small "holding area". You're let in through fast track if you wave your card and say you want to use the lounge