Personally I'm a fan of serial sites. At least, where monuments are concerned. i don't think that most individual buildings are significant enough to be inscribed on their own, but when together they make a whole that is greater then the sum of their parts.
Cities, on the other hand, I believe do not adapt so easily to being inscribed in a serial manner. Each city has its own character, its own identity, so inscribing them in unison emphasizes only the similarities and fails to highlight the differences. so unless cities are very very similar, i oppose serial nomination,
The Struve Geodetic Arc is, of course, the ultimate example of where inscribing in serial is absolutely necessary. By themselves, they would have no meaning as a world heritage site. By themselves, they would have no more significance then inscribing, say, the battlefield of Quadesh (site of the world's first peace treaty, but there is nothing there). together, the sites become much more significant.