It’s so much cheaper than ultegra, and only a bit heavier, or am I missing something?

Sort of.

It’s probably the cheapest “proper” groupset and offers the best value for money IMO.

Although, even the lower end stuff from Shimano works really well. You can get 10 speed Tiagra now.

It’s not as close in function as SLX/XT. Although how much money you want to pay for a nicer clicky feeling when you change gear is up to you.

The new ultegra brakes are really good – not sure if that tech has filtered onto new 105 but if not that’s be a big difference too.

The new 105 (out in June for model year 2014/15) looks very close in function to Ultegra. It has the new style direct mount brakes and the 4 bolt style chainset like Ultegra.

My 105 has been flawless. Really surprised me how powerful the brakes are. The gears click up and down as smoothly as any XT/SLX/whatver i’ve ever had. (Although nothing beats 1996 XT).

I’ve got the last iteration on my road bike (5600 with the external gear cables) and it’s great, it’s kinda like deore in that there’s no reason for ultegra unless you’re minted, in which case get dura-ace (or record 🙂 )

to my mind it’s the quality of the chainset – when the version comes out with mechanical shifting and hydro for disk brakes that will be the total winner for a CX-wintertrainer-adventurebike

I have 105 – it is a good solid groupset that works very well.
the throw of the gears is quite long and the shifting isn’t light or as silky smooth as Ultegra. the brakes are more than adequate in my opinion.
very reliable though.

It does seem curiously pitched in price, it seems to be a lot closer to Tiagra than it is to Ultegra in that respect. Does look like new 105 is set to close the gap in performance, wonder whether this will push the price closer to Ultegra. May well become more like the SLX (which seems to just be a slightly heavier and cheaper version of XT) of road.

Edit: by the way, I have Tiagra, 105 and Ultegra and they are all pretty damn good these days.