I've used Hotline a few times; it is good for short haul and is often great value for CE. Madrid this summer was £143 on BA.com and I got for £77 on Hotline... On longhaul average savings less, and when you see a great WT+ it can't be UuA so you will be travelling in that cabin only.

However, proactive upgrades do appear often after 2/3 days