If I were buying a new bike, I'd prefer Ultegra to 105, but I don't think I'd spend the money to do an upgrade on a 105-equipped bike I already own. To my mind, putting that money towards a new bike is going to yield a better value. My reason is that while the Ultegra is better than the 105, upgrading isn't going to give you the sense that you're suddenly piloting an F-16, whereas before you were flying a KC-35 cargo plane.

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I have a bike with each. Not worth an upgrade.
The weight difference is too small to notice, and shifting is slightly improvoed.

I would suggest that you upgrade to Ultegra as and when you need a new part due to wear and tear.

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thanks for the info fellas...it is actually just the brakes and front deraileur that are 105, for some reason the rest of the bike (bought new) was already ultegra.

That's not uncommon. Manufacturers often mix the compnent spec to hit a certain price range. In you case, they probably felt that the difference in performance between the brakes and front derailleur were small enough to swap them out. I'd be more concerned with the rear derailluer, cassette and rings than the brakes(which all use the same pads anyway). If you want to, you can probably get the front derailleur from performance for around $40. I definitely wouldn't bother swapping out the brakes. Just my .02.

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If it's only the brakes and front der... not worth the $$ or effort.