paddyd, you're right! I use the word 'desirable' a lot, mainly to give a local perspective on which areas are well regarded and which aren't. It's not always so obvious from a quick visit.
I agree with you that tenants don't really care about the address of their property once it's safe and easy to get everywhere. In Budapest though, crime is not the main issue. It's dirt! Many of the streets are very run down and dirty, so not pleasant to live on. Szondi utca is an example of this. Most expat renters I know wouldn't want to live there. More importantly though, when it comes time to sell an apartment in an undesirable location, the target market is more than likely going to be limited to other investors. I always encourage people to invest in the best location they can afford as there is no guarantee than any of these 'up and coming' areas will improve all that much.
fstruthers, I passed by Rippl Ronai 32 this evening and it's very close to Podmaniczky on the worst part of the street. The building design is nothing special and only the attic is being redeveloped and not the whole building, so not so unusual for Budapest. And although the facade and interior of the building will be renovated, I don't think it will look that great when finished. I think you might find better value elsewhere. 390HUF/sq m. is quite high for this location. Have a good luck around and don't rush into anything you're unsure of.
Best of luck,
Budapest