how useful are solo papers?
EDIT: Just to make the perspective clear. Disciplines have varying authorship traditions. In many fields within the sciences, and where I come from, first name in an author list is first author (considered to have contributed most) and collaborative papers are the norm.

Any article where you are first author is "good" for you. Single author articles have become increasingly rare, at least in fields with which I am familiar. The reason is of course that science is rarely done by single persons but rather in groups. Where single author articles are still relatively common are in the form of review articles where a single author can collate and critically assess the state of affairs in for example a subfield.

Going back about 20-30 years, again in the afore mentioned for me familiar fields, single authorship was almost demanded from PhD students and older faculty would basically frown on co-authored articles. Now, single authorship almost carries something odd about it in the sense that one can wonder if the person does not know how to collaborate. The later is of course not a good view to express but it shows how much the views have turned.

So to conclude, writing a well-referenced paper in a high impact journal as first author takes precedence over single or multiple authorship today. I doubt many would look at single authorship as much better than first authorship on a co-authored article today.