I bought a yarn kit from Tiverton bead and workshop the other day and it arrived today and here is my review so that you can see if what is important to me is important to you too;

Website - Easy to use;
I found what I was looking from from a Google search, I checked trustpilot but there were no reviews so was little hesitant.
However the site was nicely laid out and professional and offered Paypal for payment so knew I had a way to get my money back if needed.
I was pleasantly surprised.

Customer service - friendly and helpful;
I called to check on the postage options and the lady I spoke to have clear information and instructions and was very friendly.

Delivery - very quick and we'll packaged;
I only ordered a couple of days ago. When I spoke to the lady on the phone she did say she was bout to parcel up for the day's postage so it might go out same day, but I did not get my hopes up.
A couple of days later here it is, very well packaged, and exactly what I wanted.
The item I bought is a yarn kit from Scheepjes, which was no longer available from my main yarn supplier, and I was really glad to be able to get it after all. I have bookmarker their page and will make sure to check them out each time I buy yarn as it was a really pleasant experience dealing with this company.
Also, I should mention, I live nowhere near Tiverton, so was really surprised it got here so quickly.

I cannot say how they deal with problems as this is my first order and did not have any, but going from how they helped me on my phone call I would think they would be very helpful.
I would recommend Tiverton bead and wool shop (known as beadandwoolshop online) for any of your yarn needs.