Based on my experience in the UK, where there is a very heavy advertising and awareness campaign every year in the lead-up to Christmas, and where drunk-driving is far less socially acceptable than in the 'States, making DUI a felony will have an beneficial effect on the DUI rates, in a couple of ways - those who, when sober, know that they are going out for a few drinks and while drunk might be liable to drive home or offer friends a lift home, will possibly leave their keys at home; secondly, offenders who are caught and get jail time plus a driving ban (and probably losing their job as a result as well) will probably not make the same mistake again once they get their licence back.
Having said that, there is always a hard core of DUI drivers who will continue to drive no matter the punishment (although on-the-spot execution by Police would make them one-time offenders).
Most of the "enforcement" though, comes from the community and the driver's circle of friends - someone saying to that person "you have had a few, driving will not be cool for you if you get caught or for the mother and 2 kids that you kill in an avoidable accident while DUI, so we will all take a taxi home". Also, schemes like "Designated Driver", where when your group walks into a bar one person is nominated as the driver, and he gets free soft drinks all night, have been effective in the UK.
Fundamentally, very VERY few people have the necessary self-knowledge to be able to see how much their driving skill, decision-making and reaction times are affected by alcohol. Lots of people think they are fine to drive, and many DUI journeys pass without incident, but just because the driver made it home without killing someone does not mean they are ok to drive. As long as their decision-making skills are impaired, their ability when drunk to rationalize the decision whether to drive or walk/take a taxi will also be impaired, and with most people the reaction of the community around them will be the thing that stops them, rather than the prospect of jail time for being caught.