I know SIU really well. My buddy just graduated #1 in his class. Another friend of mine is a 4th year and he is #2 (last I heard). Below him is another friend of mine (3rd year) and he's #6. (they were all in my fraternity).
Anyways, I applied to SIU and got waitlisted. IMO, SIU lies about your chances of getting in. Let's face it, it's tough. 45 seats and a lot of competition. It's the cheapest dental school in IL so everybody is fighting to get in.
As for reputation, SIU is the best kept secret. I heard they rank pretty high in the boards. They have a great curriculum for sure. But, if you're concerned about the name SIU, I wouldn't go there. I'm at NYU, and although NYU doesn't score as high as SIU on the boards, almost everybody has heard of NYU and its prestigious grad schools.
As for practicing in Alton or going to school in Alton, get ready for that area. Not to denounce that area, but coming from Chicago, I'd get depressed if I was there everyday. Pickup trucks and a non-diverse population would drive me crazy. I was born around St. Louis so I know...plus, my ex went to SIUE.
But, I know that once you graduate from SIU, you'll be a great dentist. It comes down to if you'll be happy there or not. So maybe it's a blessing in disguise that they didn't accept me. If u want to practice in an urban setting, apply to UIC and get to know Dr. Johnson (if she's still there). late October/early November is the best time to get your foot in the door and show your face to the admissions committee. Schedule an informal meeting...dress in a suit. Dr. DeClue at SIU is nice, but take their advice lightly. Good luck!