I have used both Tiagra and the 10-speed Shimano groups and I would say that Tiagra can actually shift smoother than the 10-speed groups under some conditions. However the 10-speed groups have a more precise, crisp, machine-gun feel to them and perform a little better shifting under some load.

Tiagra is great value for money. I suspect that the Shimano trickle-down effect means that Tiagra has got many of the characterisitics of 9-speed Dura-Ace which was as good as it gets back in the day.