Thomas cook is a popular and well known holiday company who have been around for a very long time. I have booked through them many of times and have always been happy with their service.
The bookings process with them has always been easy from my experience and their customer service is top notch! They are always available to help and will always try and get the best deals for you. They often have flash sales and you can grab a great bargain get away with them. They often have great prices for flights, hotels and car deals.
I have not had to ask for a refund and so I cant comment on that part of their service, but all others are a 10/10 for me!