I have a 1Ds mkII and an original 5D. The fact I still have them is mainly due to the fact that they're worth a lot more to me that I would get from selling them. In other words I think they're probably undervalued on the second hand market.

The main reason I say this is that the look you get from a full frame sensor is in many ways unobtainable with a smaller sensor. I'm not just talking about noise / dynamic range. Dynamic range in good light is pretty much the same as newer bodies, noise is generally better with the older full frames but extreme low light shooting will be better on the current rebels as the analogue ISO amplification goes up to 12,800 whereas the 5D/1DsII only go up to ISO 1,600.

Instead I'm talking about depth of field, and sharpness. A really sharp image with shallow depth of field really makes the subject pop. You can't get the same effect on a crop as there simply aren't any fast wide lenses available, a 24 f/1.4 lens on full frame is equivalent to a 15mm f/0.9 lens on APS-C, the closest you can get is a 14mm f/2.8, which is over three stops slower (for about the same price). A cheap 50mm f/1.8 is equivalent to a 31 f/1.1!

The advantage in subject separation remains until 135 (well into telephoto) where you can match it with a crop, albeit with a much more expensive lens (85mm f/1.2L as compared to 135mm f/2.0L). Sharpness is better as you're not enlarging the image as much when you view a full frame photograph at a certain size compared to APS-C.

Having said that these advantages go away as soon as you start cropping your full frame images. If you are predominately chasing small lizards you may well find little advantage with a full frame camera.

With the 1Ds you also gain a top of the range 45 point AF system and weather sealing, but this comes with increased weight, confusing menu controls (you have to hold down different buttons while using the control wheels to change settings or navigate photos).

You have to consider what you're giving up, live view is certainty very useful for certain types of shooting. Rear LCD screens have improved a lot especially compared to the 1DsII, which has a tiny screen with very poor contrast. Speed of shooting and writing to memory cards in particular has improved a lot.

My gut feeling is this wont matter as you're used to not having these things with the 30D -- you will feel very at home with the 5D as it shares almost exactly the same control layout. You'll have to learn the 1Ds controls but it will reward you with much better AF.