I am a bit puzzled by what Annie's suggestion (at the top of the thread) that you can't use trade names in writing without permission and that if you do so you could get into legal difficulties.
I'm not a lawyer, but in my limited understanding, the answer lies, as always, somewhere in between, and AFAIK applies roughly the same in the US and Germany:
1. Companies can not control how people spell their brand names or what context they are used in in day-to-day life.
2. Trademarks, however, are a legal basis for companies to prevent competitors from using the same name for competing or at least similar products. And importantly, "similar" to whatever kinds of products the term was registered for with the relevant patent and trademark office, not necessarily just for what you know that term to stand for. E.g. many companies register their terms for use with "computers" as well simply because they plan to have a web site.
So use in business, advertising or sometimes even just public forums to describe your home-made play dough using the word "Play Doh" may be grounds for a trademark cease-and-desist order, unless it is sufficiently clear that you're only referencing the trademarked brand as an analogy.
3. Publishers (even of novels) have their own style guides about how trademarked words are to be used. It seems to be mostly an agreement between publishers (who trademark book titles and character names, too) and big brands to avoid that whole thing where a term falls into common use as the generic term. If all book publishers uppercase their use of Kleenex, then it becomes much harder to prove that "kleenex" is now the generic term.
(If stuff like that interests you, have an eye out for the "John Carter of Mars" book series. Its copyright recently lapsed, but the Edgar Rice Burroughs Estate still holds trademarks on the titles and character names, and got into various lawsuits with comics companies who made comics based on the now public-domain stories, and so obviously had to use the character names ... it's interesting which side wins in such a case, and what the lawyers argue).
4. While the general laws for what can be trademarked are very similar, trademarks are per-country. As such, while you can not trademark a simple German word in Germany, Switzerland, Austria, Luxembourg and Liechtenstein, a Spanish trademark official may not have the same qualms (even if they're aware of the word's existence in the other language). Similarly, there are e.g. German companies that have trademarks on generic English terms. This makes for some fun international kerfuffle on the Internet, for instance (anyone remember that German company named "GMail" that got the web site of Google's mail service blocked in Germany?)
5. If you write in a negative way about a brand name, you have to be careful to not exaggerate and to clearly mark subjective opinions as such, and that you're able to back up your claims sufficiently. Otherwise you can run afoul of libel laws. Thus, if you're not actually talking about the trademarked brand, but the general class of products, it's usually better to avoid mentioning their name.
So, if you know this and apply common sense, it's usually fine to use trademarks in normal conversation, but it helps to be aware of the words that are trademarks if you're writing in any regular or professional way.