I acquired an MT-640 as a "hallway score." Someone in my building was throwing it out, so the price was right. That was about 4 years ago and the more I use it the more I like it.
I haven't updated the cartridge and/or stylus yet, what it had has been fine so far, but I'll soon get that taken care of and let you know how that went.
I'm not a total enthusiast for top-end sound, my whole objective was to have a "record player." At least for now. The rest of the "system", such as it is, is a Creek moving magnet preamp (OBH-18 I think), and a Tivoli Henry Kloss Model two with the sub-woofer. IMHO, this makes close to an ideal record player. It sounds very good.
I can't hear any wow, flutter, or rumble out of the '640, and so far the mechanism has worked flawlessly. I set the stylus pressure to 1 gram and so far, so good. I haven't really explored how low I can go with it.
I have plans this weekend to take the cartridge shell to a high-end place near me (where I got the pre-amp and Model Two) to get thier input on what to do with the cartridge or stylus.
Hope you're enjoying your '640.