Is it legal to use "sucks" in a URL to complain about a company?

There is a such thing as slander, which usually covers spoken words but can extend to written ones as well. Making a claim "ABC Sucks" could be construed as an opinion statement, but if anything is said about the company that is not 100% true, you might be setting yourself up for some bad news.

Consider this: if ABC, or any company out there for that matter, turns out to do dirty deeds, they may do whatever they can, including having a lawyer comb your entire site, fact-check every single thing you post, and they may perhaps even hire people posing as fans to try and egg you on into breaking the law. That and if you post any pictures on the site that are obtained through any means other than 100% legal procedures, or used without the companies consent might be handing them ammunition. For example, if you went and grabbed their logo from their website, they might try and sue you for using it without permission.

I suggest that if you are intent on taking on a giant like ABC, being one man/woman yourself, you better have a warchest of money just in case you actually have to go to court. Don't depend on some free speech advocacy group to rush to your defense. Don't expect to rally any prospective fans of the site to protest on your behalf. The most they would probably do is blog about how unfair it is and be done with it.

So if you are still intent on doing what you described above, I would suggest creating a concept site on disk and bring it to a lawyer to review. You will have to pay him/her for their services but trust me, it will be worth it. They will go through the information you have up there, and tell you what, if anything, you can be sued for. Ask this lawyer for some general guidelines to stay within the law, and then pay them for their time. Hang on to their business card because if you receive a "Cease and Desist" notice from ABC, that's the first person you are going to need to call.

And that's the best advice I can give.

Free speech doesn't protect you from them suing you. It also doesn't protect you from violating their trademark.

I know you think this is a great idea right now but you need to realize that should they decide to sue you, even if they don't win, you could be ruined by the cost of defending yourself.