I used, well still use, the 3.0 system only with the numbers generally halfed.
So usually what you have is something like
DR 5/+1
DR 10/+2
DR 15/+3
DR 20/+4
DR 25/+5
Also 'cold iron' in my system basically means 'quenched and tempered steel', which is the ordinary material which weapons are usually made of. Steel itself has anti-magic properties because it is 'unnatural' (doesn't occur naturally, unlike for example copper or gold). So pretty much any weapon usually overcomes at least one sort of DR. Also, the special materials are themselves 'true' instances of common materials, so Adamantium is 'true steel' (and overcomes 'cold iron' DR) and Mithril is 'true silver' (and overcomes 'silver' DR). Also, aligned materials overcome their opposite as well, so a 'good' weapon overcomes both good and evil DR. This reduces the size of the tool bag needed.
The problem I have with the 3.0 system RAW is that it effectively is the 1st edition system, in as much as if you don't have the weapon for the job, the target is effectively invunerable. The problem I have with the 3.5 system is that it usually depends on having a golf bag of weapons you pull out for a particular situation, but that the straight DR/magic system is pretty much irrelevant in 95% of situations because magic weapons rapidly become ubiquitous and usually before DR/magic creatures are encountered.
My basic goal is DR which heavily rewards having the right tool, but which doesn't provide complete invunerability. By the time you encounter DR +1, you either have +1 weapons or can in a pinch do more than 5 damage. By the time you encounter DR 15/+3, you either have +3 weapons or in a pinch can do more than 15 damage. Generally I like to throw an occasional encounter at the party which would be easy had they the tools, but which becomes challenging and encourages creativity without them. By using low numbers for DR, I can do this without having a monster that is completely immune to attack, so if the players don't get creative I'm not throwing them into a TPK.