Same here -- the 5 or so you can download for the 6680/6681/6682 are top notch, when I used that phone they're all I ever liked day-to-day. Same with the new S60v3 default theme, it really stands up well to constant daily use with all the different features. A lot of 3rd party themes look nice and work well, but there always seemed to be one or two oddities that irked me and not want to continue using them.
It makes sense -- our company, like Nokia, employs both high-end UI designers (the lead has a PhD in Psychology, is a huge 'colour theory' nerd and has studied how colours work inside the eye and how they cause feelings to occur) as well as top notch Graphics nerds who work with them to create pleasing themes. A single person making their own 3rd party theme is hard pressed to do all that on their own...