Here's a tip to consider if your pet is ill and you plan on taking him/her in to see a vet: Ask for a morning appointment vs one later in the day.
Why? The closer you get to the hospital's closing time, the greater the chance you will be referred to an emergency clinic for diagnostics and treatment if your pet is suffering (or is thought to be suffering) from something serious. Most pet emergency clinics open at 6PM, so appointments made on or after this time are at risk of referral. Of course, if your regular vet operates extended hours (say, until 10 PM), this won't really be a problem.
As you might expect, most pet emergency clinics charge much more for diagnostics and treatments than do daytime clinics, so your pocketbook will definitely take a hit. On the other hand, if you take your pet in for an early appointment at your regular vet's office, the costs should be less since diagnostics and treatments can be done (or at least started) right then and there.