I have been using gymshark as my sole provider of gym-wear for 4 or 5 years. I estimate that I have spent around £1500-£2000 in this time. However this extreme amount of money is not because I have to! I still own my very first pair of gymshark leggings and although they have faded slightly, they're still excellent quality.
I have consistently used discount codes and sales with success, however it is worth noting that during their blackout sales it is almost impossible to get through checkout due to the demand.
Their customer service is excellent and consistently helpful. The only problem I have experienced is some inconsistency between sizing.
I recommend Gymshark.