Since my first tentative but very positive use of the site in Aug 2015, I have had only wholly positive transactions. I cannot fault the site and I've saved loads of money shopping with them, as a UK customer. I'm so glad I bothered to take a calculated risk and realised that (most of) the negative reviews were due to people not doing their homework and ignoring the customs guidance which is quite clearly stated on every order summary page. I cannot overemphasise how easy and positive my experience has been. iHerb are my first port of call for supplement orders.
All you need to do to be successful is a bit of straight-forward calculating to ensure you are getting a better price from iHerb after the custom charge (20%) and the handling charge (c£8 for normal sized orders) - very simple to estimate! - than you would ordering it at a more expensive base price from a local retailer.
As a UK customer, you may be charged 20% of the value of your order in import duty/customs tax. You may also be charged a flat £8 handling fee. That's all you have to work out!
So, when once everything you want to order is in your basket, look at the total cost (including delivery, but after any discounts) and add 20%. Simplest way is to multiply the total in £ by 1.2. Then add £8 to this. If that figure is less than you would spend ordering from elsewhere, you're quids in. It is rare that I can beat iHerb for price.
My referral code (you will get one too, to give to others) will get you $10 off: it's SPM026.
Honestly, if the prices work out better on iHerb, give them a go. From my experience it's been easy, straightforward and provide top quality products for a really decent price discount. Can't fault them at all!