Wow! Thank you guys for all your helpful replies - I'm genuinely impressed by the advice you've all given.
i use the 100mm L with a tripod and slider. i use the slider to zoom in/out and/or focus.
^^^ I didn't even know that such things existed... if I end up going with the 100mm, then I'll definitely get a slider.
If small product photography is important to you, I'd suggest not getting a macro lens, but a tilt-shift instead. The TS-E 90mm f/2.8 is ideal for shooting small products. The problem with a macro lens is depth of field - at macro distances, DoF gets really thin (as little as a couple of millimeters or less), so you need to use a very narrow aperture - that means you need lots of light, but also, at narrow apertures your images get soft due to diffraction (starts costing you sharpness at about f/7.1 on 18 MP APS-C or f/13 on 21 MP FF. Tilt gets you the DoF that a macro lens can't deliver even stopped down way past the point where diffraction results in a soft image.
^^^ Thanks neuro - I've always loved what can be done with TS lenses, but could not justify getting one - however with your explanation and then seeing the examples you linked to the TS is looking like a good contender

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lol... Maybe I should clarify what my uses actually are:
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The objects I'm photographing are items measuring between 5-10cm in length.
The camera I used in the past was a 350d with a sigma 28-70 f2.8.
The sigma has a reasonably close focusing distance and being a crop sensor, I got a bit more magnification from this setup (the long end would be equal to around 112mm I guess) - therefore I could fill the frame reasonably well.
I recently upgraded to a 5d mk2 and the 24-105mm, but I'm finding that on some of the smaller items I am unable to get in close enough to achieve the effect I want - I just need to be able to get a little bit closer than the minimum focussing distance of the 24-105.
For instance sometimes I want to photograph maybe a 2-3cm section of a item, (often at an obscure angle) using f2.8 to get a specific look, but the 24-105 just falls short.
With my previous setup (the 350d and 24-70), I could set up on the tripod, take a 30mm shot, a 50mm shot, and then zoom to 70mm and get the same shot close up (this is why I was asking if there are any macro zooms).
I don't know if thats necessarily the best way of doing things, but I found it worked well for me and I had a good workflow using this method.
I guess all I need is a lens that has a closer focussing distance, and not neccessarily a macro lens.
However having said all this, neuro's suggestion with the TS lens might actually generate better results (even without all the zooming in and out!) - so I'm definitely going to look into the TS lens a bit more.
It was mentioned that lighting might be an issue with the TS lens - I have elinchrom flash heads and softboxes at the moment, however I think I'm going to sell them and get some continuous lighting units instead - would this be enough lighting for use with the TS lens?
Incidentally, I did consider extension tubes as well, but I get the feeling that I'd end up constantly removing and replacing extension tubes - or I'd have to take pictures from 2 metres away!