We have used Viator for two of our big holidays thus far and our experience has been very positive. Sadly, however, our most recent holiday did not get off to the best start as a result of a Viator booking. After a two hour delay in immigration upon arrival at San Francisco we purchased a US SIM and proceeded to phone the private vehicle transfer company as per the voucher booking instructions only to find they had left without us. This transfer had been booked and paid for, at time of booking were were asked to include flight details and arrival time. The long delay in immigration, however, was beyond our control. After 26 hours of travel we were understandably upset at having paid for a transfer that was not there waiting for us as expected and consequently having to organise a metered taxi as alternative transportation. I strongly feel that the supplying company should give more detailed instructions to ensure that other Viator customers do not end up in the same predicament. Thus far, as previously mentioned, apart from this disappointing experience, all our other Viator bookings have been fabulous.