I have an Ultegra bike and a 105 bike- both are nine speeds. The 105 has 5000 miles on it, works great, seldom needs tuning. 1000 miles on the Ultegra so far, great as well.
Next bike will likely be Ultegra, simply 'because I can'. But there is nothing bad I can say about my 105- it's great too, and very dependable. Frankly, I don't notice a big difference between the two.
I've read that there is a bigger difference/jump in performance and quality between Tiagra and 105 than there is between 105 and Ultegra.