Having taken the plunge recently and bought a Digitrax Zephyr, I've set about installing decoders in my locos. The collection of stock is motley and goes back over many years, the diesels in particular being, shall we say, venerable so the less said the better.
I bought the small Digitrax DZ143 decoders for my steam locos. Actually the first embarassment was my "DCC-ready" Bachmann Hall class (32-003) which had a socket for the decoder but almost zero space. However, when I'd decided to remove redundant suppression circuit (a couple of capacitors and inductors) there was space (but only just enough!) The moral is: just because it says it's DCC-ready doesn't mean it's going to be easy to fit the decoder.
My older 93xx mogul (Bachmann 31-801) definitely wasn't DCC ready and was the hardest to fit I suppose, although it didn't really feel like it. This being a split-chassis design, I had to isolate the motor from the chassis, filing off a small amount of metal around the motor and fitting insulating tape where necessary. I stripped the chassis to all its bits and filed off enough space from the two steel halves under the loco's dome for the decoder. I brazed soldering tags onto each of the two chassis halves for the decoder's read/black wheel connections. The rest was quite easy; it all worked first time so the advice I'd read on the Web was obviously pretty good.
My Hornby 27xx pannier tank should have been easier but actually was harder than I expected. It has a single chassis so the modifications were simpler in principle. However, isolating the motor from the chassis proved to be tricky. And space is in very short supply. But in the end it went together and works well.
The Diesels were generally easier because there's much more space for the decoder. For all of them, I bought DH163 decoders, which are high-spec and yet not very expensive. The Heljan DCC-ready Class 47 was as easy as it's supposed to be - plug and play in its most literal sense! (Well done, Heljan!) The others were trickier, mostly coz of being old and having inadequate power pickups on the wheels, so I modded them with extra phosphor bronze springy strips to improve things.
Only one of the locos has lights, these being incandescent. Digitrax correctly advise adding a small-value resistor in series with such bulbs. I didn't do this at first and the decoder immediately shut down due to the current surge. Adding the recommended resistors fixed it quite satisfactorily.
Lastly, we have a new-ish Class 170 DMU (Bachmann 32-452). Although there's plenty of space inside for the decoder, it was quite tricky to alter because of the need to wire the headlights, of which there are three (at each end). A circuit board is fitted to this model which has a pair of diodes for directional lighting on a DC system. With DCC, this is obsolete but hard to remove because the plastic structure was glued in place. To mitigate the risk, I simply snipped the wires and left it there, soldering the decoder to the snipped ends instead. This involved quite fiddly soldering inside the plastic model. The remaining problem is what to do with the trailer unit, which also has lights controlled by diodes and therefore has lights that don't work correctly on a DCC layout. The two obvious alternatives are to put umbilical wires between the two units - which I'm really not keen on - or buying another decoder (anything cheap with two function outputs will do). I think I'll get a DH123, Digitrax's cheapest, via airmail from the US (like the rest) - unless someone has a better suggestion. The US prices are very good for Brits at the moment: if anyone wants to share postage, let me know.
All the decoders have the high-frequency drive technology (Digitrax call this SuperSonic). Therefore I removed the RFI suppression capacitors from all the locos, because these would otherwise waste some of the energy from the decoder.
To summarise, the experience was enjoyable - if you like making and mending things (which I do and I guess you do otherwise you wouldn't be reading this!), mostly because the outcome was most satisfying. I really think that switching to DCC was a good idea.