I have mixed feelings bout GES. I've used them a few different times for various events only because I didn't have another option. GES is the only trade show contractor I've ever used before so it's difficult for me to compare. I prefer to do most of the shipping and handling myself but some events require you go through GES.
My materials arrived at the event damaged but I knew I didn't pack them as best as I could, so I left that one alone. After I received the materials back at the office after the event, they were damaged again, and the boxes looked like they had been run over. A top to one of the boxes was completely missing and replaced with tape.
The prices seem high for the materials you have to rent. For a particular conference, we were required to rent carpet squares which cost over $300 for 10x10 area. I also needed to rent an easel to replace mine that had arrived broken, and a hightop table as well. I had invested over $1,000 through GES and this was before any shipping.
I can say I had a very pleasant experience with customer service. I had to rent some furniture last minute and they were very accommodating. The return shipping process was made very easy with their support, and everyone I spoke to was very kind.
I don't think I go to enough trade shows to fairly say if they are the best or the worst. I'm iffy with their prices but their customer service is great.