Hatyai is best for mid-town atmosphere, not such a big city like Bangkok, but with all the modern amenities inexpensively and conveniently available. Chinese foods there are abundant and good, much better and cheaper than Singapore. Thai versions of Malay foods there also beat those in Malaysia and Singapore hands down.
Ironically as the name Hatyai means big beach in Thai, but there's no beach nearby. If you're looking for beach, it's tens of miles away in Songkhla, about an hour's drive. Not a much of a beach to shout about too, but the seafoods there are cheap, fresh and good. Good for a lazy afternoon of seafoods and beer. Don't expect Phuket standard and quality of sand, sea and scenery there though, perhaps Changi beach.