I've had some split screen clones in both my K20D and K10D. Ebay items. Problem is that the split screen doesn't always give an accurate indication of focus - this is because, apparently, there are some very thin shims factory fitted to the focusing screen carrier that adjust for the manufacturing variations of camera and screens. So when you fit another screen it might not be in exactly the right place. I got this information from KatzEye themselves, but thankfully had only spent £17 on a Chinese copy
That said, they are eminently usable and easy to swap when required.
As for focusing older lenses, if your eyes let you down a bit, what about the in focus 'beep'? I get that on both my existing bodies with manual focus lenses, and the focus point flashes momentarily too - with my much loved Chinon 50mm f1.7 for example. I admit I only use spot focus.
I got my 'cheap' F series 50mm f1.4 also from eBay, just under £200.
It'll be interesting to see what the 35mm 2.4 performs like when it (eventually) arrives from the UK.