The commissary typically has the lowest prices on food compared to any grocery store off post (including Wal-Mart), plus the lack of sales tax adds to your savings (although there is a 5% surcharge). Prices on meat are often over a dollar per pound less than at Wal-Mart; prices on items like soda and crackers are closer to what you'd pay off post. It's been rare that I've found any food cheaper off post; all that comes to mind is Wal-Mart's 50 cent bread (which I believe is a sale price).
However, the store is absolutely crowded on payday and payday weekends, especially at the first of the month and on holiday weekends. Lines can stretch to the back of the store. Definitely AVOID grocery shopping near payday if you can help it. The store is much easier to navigate and checkout is a breeze on other days. The commissary was recently remodeled somewhat (new floors seems to be the main change) but I don't see any change, really. It still looks like it could use a makeover.
Remember, too, that baggers work for tips only and do expect to be tipped for their work. This does subtract from your shopping savings a bit, but even so, the prices are the best around.