16-45 DA wobbly front element and poor focus

Sorry, I seem too have dropped this into the wrong forum, feel free to move it.

I have been trying to work out why I hate my 16-45 and everyone else raves about them. I did some tests using a tape and matchbox and concluded that it rear focussed. took it to the shop and it came back from Pentax 'fixed' (apparently), so I took some more shots - no change.
When it came back from Pentax, the lens seemed to rattle and the front element is really loose - about 2 or 3mm movement in every axis including in and out. Maybe it has always done this and I hadn't noticed.
I think the lens has not been fixed at all, because it almost never produced sharp images and still doesn't. But I will get back to you on this.
Yesterday in the park, shooting my 1 yr old - bright sunlight, F4 mostly, wide angle mostly, some shots composed in the view finder, some just snapped at ground level. about 2 thirds were out of focus despite having loads of contrast and being in the middle of the frame. This has been my experience for about 10 months. Is this lens really slow to focus? or really prone to focussing on the wrong object?

[edit] I have just done some tests and I am certain I have a dodgy copy of this lens. every shot is focussed behind the object and wide open nothing is focussed at all. So I just need clarification on how 'tight' the front element should feel.
ta

My 16-45 is allso a little wobbly, I believe this is designed this way.

Regarding your focus problems, they are more likely caused by the camera because the lens doesn't determine focus, the camera does that.

If you search this forum you can find a method to get adjust the camera for front/back focus.

Thanks Golfdiesel (is there any other kind!),
This is the only lens of mine which does this. I tried the tamron and m200 (using the beep) at the same time and both focussed exactly on the bottom of the match box sat next to a tape measure. As a second test, when walking around the park I focussed on several high-contrast objects and the 16-45 either back focussed on 45mm or everything soft at 16mm

My 16-45 actually rattles as I walk never noticed this before.

Actually I need to eat humble pie and say that after much more rigorous testing using that printed chart and a tripod, all three of my AF lenses exhibit a degree of back focus.
The 16-45 is about 30mm behind at 45, and 40 to 50mm behind at 16.
The 10-17 fish eye is roughly the same, although I am not sure it can do this test really.
The Tamron 70-300 is just in focus at 180 (macro) with the best bit behind, and at 70 it's difficult to tell because it's soft and fringed! but essentially about 30mm back focussed.

The Winner The SMC 50mm M 1.7 absolutely sublime! bang on and you can see the paper grain. I ignored the view finder and waited for the indicator lights. (although my eyes 'agreed' with it).

So does this mean I have a camera body that needs adjusting? and if so how do I do it? and can it cope with the differences? - the 16-45 really is appalling wide open.