The biggest problem with buying a bike at one of the large box retailers is that you pretty much have to assume that the bike is not put together right. Any time I've looked at one there are always glaring problems with assembly. These range from brakes that are installed backwards to basic adjustments that are so far off that the brakes won't work right. If you don't know anything about tuning up a bike expect to pay at leat $60 to a LBS to adjust things. The sub $150 bikes also come with realy low quality components. I've seen folks with major crank problems because they were not installed right. Others found that upon taking their bike appart for reassembly (because the didn't trust the 1st assembly) found many of the bearing surfaces to be completly devoid of any lubrication.
The bikes I've seen at Target that see for aroudn $180 are much higher quality and are about the same as the $250 bikes that you can get from a LBS. The same issue of bad initial assembly applies to these bikes. If you figure the cost of taking the bike to a LBS to have it adjusted, the cost savings are not there. After a few weeks of riding you should have the bike readjusted as the cables may stretch a little as they get some initial use, your local bike store will do this follow up tuneup for free. For some basic riding, a lower end bike from your LBS will work just fine. You don't have to drop $1000 to have some fun on a bike.
Unless you are mechnically inclined, and know what deficiencies you will have to put up with, I would generally avoid the sub $180 bikes. You'd be much better off finding a used brand name (Trek for instance) bike in your local paper or Craigslist for a lot less and have a bike that is of much higher quality.
Happy riding,
André