I've had HUGE problems with Wal-Mart and from a number of different perspectives:
1) As a customer: The quality of products are usually sub-par compared to competitor offerings (not always, though), the shopping experience stinks (this is almost universal) and the patrons often make me yearn for a banjo to pluck away at while shopping there.
2) As a vendor: Worst strongarm tactics in the industry, they treat people HORRIBLY on the vendor side, their demands are nothing short of putrid and the majority of managers and staff at their corporate office are complete and utter idiots when it comes to making business decisions (I've had this validated by numerous ex-employees that worked in FayetteNAM and BentonHILLBILLYville). Their corporate culture is one of "beat the vendor to s**t" and it's the worst I've seen...
It seems like their corporate office likes to hire their "rank and file" with the lowest education levels they possibly can to keep pay down and advancement at a rate that they set. I had a friend that worked his ass off for them in accounting and was treated horribly. After he got his degree, he gave them the finger and has never looked back.
Furthermore, the office locations I've visited are hit or miss. One particular area had moldy carpet that covered most of the office area and cubicles that were akin to working in a trash can they were so small.
I swore long ago to NEVER badmouth a customer; Wal-Mart is the SOLE exception to this rule.
3) As a supplier: Worst and nastiest negociating tactics I've ever seen, exhorbant demands made from the beginning and they do their damndest to control your margin and supply chain inside and out, no matter what the cost to the supplier. Demands only get more exhorbant as times goes on, too. Inventory issues or quality issues that are normal when starting up? Kiss your ass goodbye, as their legal dept. will EAT you alive if you are a budding business trying to get off the ground. Long story short, if a large portion of your business is Wal-Mart, they OWN you.
4) As a concerned citizen: Along with a number of other citizens in our area, I petitioned against Wal-Mart building a SuperCenter a few miles away from us. It doesn't effect my property values or cause any nasty traffic for me, but it does threaten some of the smaller shops in our area. My former next-door neighbor was on the petition council and during the city council meeting was verbally accosted and threatened by a pair of lawyers that represented Wal-Mart in this matter. Long story short, he was told that he either change his tune or get used to litigation...
I'll burn in HELL before ever stepping foot in a Wal-Mart or patronizing ANY part of their business. They are the PRIME example of capitalism gone amok and they day the US government starts looking at them in terms of anti-trust is the day I pop the cork on a bottle of '94 Opus One I've got stowed away and party HARD.
There are other companies out there that use some of the same nasty tactics and hold the same corporate culture, but Wal-Mart is by far the worst I've dealt with and one day may their stock be delisted.

[Deep breath]
Ok, I feel much better now.