Is public transportation that inexpensive where you all live? I'm only 7miles from work, and commute by bike (weather permitting). The bus doesn't really stop near my office, so I would have to walk or ride a distance anyhow.
I fortunately live literally across the street from the grocery store, and unless I'm getting a whole load of stuff (including dog food, cat food/litter) I walk or ride - my panniers carry more than my arms
We also live within walking or riding distance of our favorite restaurants/coffee shops/train. But the train can cost more than driving. It's $14 round trip for DH and I to go downtown; its only 15 miles one way and doesn't cost that much in gas to drive that far. We usually use the train for convenience like when parking will be a hassle (sporting events, festivals, or concerts). And even at 15 miles one way, we'll take the bikes to an event depending on what it is - Coors Field has secured bike parking for Rockies Games, and B-cycle offers secured bike parking at festivals - so if the weather is nice, and it's okay for us to show up looking like we're riding bikes, we do that.
I've never taken the bus here, but the train is just too expensive to use on a routine basis for the trips we take.