Yes, I've been investigating my Schuessler relatives of late, and have experienced the same variations. Even though it has been a while since your post, I thought I would explain why it is that way, in hopes of helping you or perhaps others researching Schuessler relatives.
The original spelling in German would be Schüßler. You will note that the name as two characters in it, which we do not have in English. One is the umlaut over the "U", which you already pointed out, the other is the ß, which is called an "S-Z", "scharfes S" or a couple of other names in German. (If you're interested, there's an article about the character at http://en.wikipedia.org/wiki/%C3%9F.http://en.wikipedia.org/wiki/%C3%9F.)
There are conventions for writing words with umlauts and ß in scripts that do not have those characters. An umlaut is changed to an "e" following the character that is under it, whereas the ß is written as "ss" or, formerly, "sz", as the name implies. (I think the "sz" disappeared with the switch from Gothic to Latin script in official German printing, but I'm not positive about that.)
So, that gives various possibilities for writing Schüßler, namely Schuessler, Schueszler, Schüssler and Schüszler, depending on whether or not the typeface has umlauted letters and whether following the sz or ss substitution for "ß".
But those only cover the case where someone who is familiar with the rules is translating the written name. It gets even more complicated when you consider the fact that those writing the name might not be familiar with German, so they would just write what they hear. Depending on the regional access of the speaker and the language knowledge of the listener, you might get any of the above possibilities, or the umlaut might just get dropped, as might one of the "s"es. "ü" can also sound a lot like an "i" (which gives us e.g. Müller -> Miller), so you can even get Schisler or Schusler or ... And of course the "c" gets dropped sometimes, also, leading to Shussler, Shisler, Shusler etc.
Dealing with different spellings of the same name can certainly add to the challenges in genealogical research, but if it were easy, it wouldn't be nearly as interesting, at least for me. :-)