I've had both factory service manuals and Haynes manuals, and found the factory manuals to be superior. Mostly it's a question of production values. My service manuals have always been printed on decent paper, with clear line drawings done by professional illustrators. The Haynes manuals I've used have been printed on what looks like a grade of newsprint, with murky photographs depicting the various procedures, rather than line drawings. No doubt these are all cost saving measures to keep the manuals in the $30.00 range, and the manuals are good for many procedures, but they do lack some detail for more complicated stuff. I've bought factory manuals for both my V-Strom and Super Tenere; they're definitely pricey, but I've found them to be worth it.
Like Hans says, they're overkill if all you want to do is change your oil, bleed your brakes, or maintenance tasks like that. There are lots of Youtube videos on how to so those things. For something more extensive like removing the steering head, or replacing a seal in the final drive, I'd rather have the official service manual with lots of clear illustrations.
"No matter where you go, there you are."