I recently used Giant Ibis to travel from PP to SR. I only have good things to say about this company.
The coaches are new with good aircon, VERY good legroom, they show recent western films and you can turn the speaker off above your head.
They came to collect me from my hotel in an equally new mini-coach. The places we stopped for rest breaks were clean and pleasant. Only downside was no onboard toilet.
This contrasts with the experience i had with Mekong Express from HCMC to PP - very cramped uncomforable coach which overheated and broke down.