26" Schwinn Sidewinder $149 26" mtb, shimano 3X7 gears.

This is the best bike you can get at walmart for $150, if you are a medium sized person.  Every magna, roadmaster, mgx, huffy, next bike that is cheaper than this is a total piece of garbage.  This bike is actually serviceable, functional, and worth fixing.

The Schwinn Sidewinder is a 26" wheel mtb, with 3X7 shimano parts for 21 gears.

It's parts are pretty decent:

Shimano tourney 3X7, derailleurs and grip shift shifters.

Alloy linear pull brakes and levers.

Wheels with alloy double wall rims and alloy hubs.

It compares pretty favorably to a Trek 820, $380 bike shop quality bike.

http://www.trekbikes.com/us/en_US/bikes/mountain/sport/820/820/p/1008600-2016

The gears, wheels, brakes are the same.  The trek does have better tires, thread-less fork, nicer bars, seat, post.  And it comes in many sizes.

This gets to the main thorn with the walmart bike.  It only comes in one size, 17", or medium, good for people 5'7"-5'10" about. If you are shorter or taller, you are out of luck

The bike shop bike has 5 sizes, XS, S, M, L, XL.

Also, the bike shop bike is put together by someone that knows how to assemble and adjust a bike.  This makes a huge difference!  If you took the Walmart schwinn to a bike shop to have them assemble/tune the bike so it works right, labor would likely be $50-75, and worth it.  That said, bicycles are not rocket ships, and someone with good mechanical aptitude, and willingness to do some research and buy $25-50 of tools and lube can do a good job.

The main crappier parts about the Schwinn is the threaded fork and stem.  And crappy suspension fork.

The handlebars and seatpost are also heavy, cheap steel, where the trek are aluminum.  

Neither of these are that noticible to basic commuter bike rider.

The tires are a weakness.  The stock tires get flat easily, and wear out fast.  Replacing them with flat resistant tires would be a huge upgrade for someone using the bike to get to work.  The knobby tires roll slow on the road.

$13 Panaracer 2" touring tires are a good choice

http://www.niagaracycle.com/categories/tires-pan-tour-26x2-0-wire-bk-bk

Otherwise, just use the bike til stuff breaks.  Most likely things you are likely to run into issues with are the chain. $7 KMC Z50 chain http://www.niagaracycle.com/categories/kmc-z50-chain-6-7-8-speed-1-2-x-3-32-x-116l-silver-brown

Seatpost.  Check the stamp at bottom of post, likely 27.2mm, and get alloy Kalloy seatpost, around $20.  

For people that are not medium sized, or people that live in areas that have good used bike markets, a used 90's MTB from Trek, Giant, Specialized, Cannondale, etc, for less than $150, is a better deal.