I've currently had 2 orders with SixMM and a 3rd on the way, totalling to about £1000.
They take a few weeks to just send the order - and it seems a bit of a roulette from there. Thankfully, they mark the value as low on the packages, so I didn't have to pay anything extra upon arrival to the UK and it slipped straight through customs.
1st order:
Messaged them beforehand to ask if a particular item was in stock. Bought it, turns out it actually wasn't. That delayed the order by a month. Order eventually arrived and was mostly right, except they sent a standard curved trigger instead of the flat trigger I ordered. Sent them a WhatsApp message & they asked if I'd be okay to pay another $40 for the flat trigger which I was originally meant to receive? Told them no as it was their mistake and that I'd want it sent with my next order, which they did.
2nd order:
Again, mostly correct, except it was missing the main item (GM MWS bolt), but they didn't tell me that & so instead had to wait the few weeks again for them to send it & the ~2 weeks for it to arrive to find out... One item's plastic packaging was shattered but didn't really matter as it was just a foregrip. As mentioned previously, they included the trigger from the 1st order that they originally didn't include.
Told them I just wanted a refund for the bolt as I couldn't be bothered to wait again, which they did, thankfully.
3rd & probably last order is currently processing, so will be update this in 2040 when it arrives.
Their prices are great, processing time is abysmal, site stock tracking is hit & miss, communication on order issues is non existent until you bring it up.
Update (9th June 22):
Order 3 was placed on May 26th, got it today and used their cheapest shipping option, so not bad. However, I ordered a 260mm 6.04 Crazy Jet inner barrel & they've sent me some weirdly cut stainless steel 230mm barrel which is marked "6.03x650mm", suggesting it has been cut down, and then placed inside old beat up Maple Leaf packaging. Curious to see what they say.
Only use SixMM for non essential items. They have screwed up the important items in every order so far.
Update (13th May 23)
4th Order:
They've definitely improved, especially with customer service's response time. An item in my order appeared to have been used & was missing a piece, but they contacted the manufacturer on my behalf & got them to replace the missing item (took about an additional month), so quite happy with how they handled that. Though, I'd prefer a brand new & complete item to be sent in the first place. Initial shipping wasn't too bad, usual few weeks. Tracking number didn't update beyond leaving HK though, so turned up unexpectedly, but I've noticed that with various HK orders recently.
Yet to have a perfect, straight-forward order, but their stock selection is often unique & their prices are great.