I am giving Mountain Warehouse three starts. This is mainly based on quality of items. The prices are however cheap and if you aren't a seasoned hiker and just need something cheap and cheerful you'll be fine. If you need things to actually hike a mountain an anything other than perfect weather then give this a miss and look for a well known brand such as Rab, North Face, Mountain Hardwear/Equipment and Icebreaker. Granted there is a big price difference but you will have a quality item that lasts. I would compare Mountain Warehouse to brands such as Karrimor you get in Sports Direct.
You will always find discount codes, once one offer expires there are always other offers. It's like they have an endless sale on so prices are always low.
Delivery is often free with codes or over a certain amount and it's generally prompt and without issue. I've never contacted Customer Service so can't comment on service but items I've had have not lasted well.