I had a chance to use Rupert Neve Designs 500 series modules in my studio. The modules I tested were 542 Tape Emulator, 543 Compressor and 551 EQ. I probably should say that I am no fan of 500 series format at all, due to the size. The modules are too small to fit in features I usually expect from audio equipment. Having said that, RND modules are exceptionally good. I particularly enjoyed the compressor and the EQ. I didn't enjoy tape emulator much. This isn't something I want for my work anyway. I just didn't care about it much to be honest. 551 EQs turned out great when I recorded a band a year ago at The Rolling Tapes Studio where I had 7 of those at my disposal. They present themselves equally well in the mixing environment although I have to say: You can't expect them to replace SSL X-Rack EQs. They could only compliment them as an additional EQ stage. SSL versatility and recall is hard to beat. 543 Compressors are very punchy and work great on drums. I still enjoy 1176 and LA2A more on most things but it certainly wouldn't hurt to have them alongside mentioned classics.
Here's the picture of the set I tested: