Essentially both the YDP640 & YDPV240 are the same as far as controls, sounds, and electronics with one being portable and the other in a furniture style cabinet. The YDP181 (which should also be replaced fairly soon based on my best guess because it's been out for awhile) is a much more basic piano although it does have a better key action and piano sound than the other two including the newer versions. Yamaha makes reliable and fun to play digital & acoustic pianos and for many years I have played them professionally and enjoyed them very much. They are generally very well built and have a big following among musicians and families. The Yamaha company started in Japan in 1887 founded by Mr. Yamaha, and built their first Japanese piano in 1900 (over 100 years ago). They were a piano company long before they started producing motorcycles, wave runners, ATV's, snowmobiles, tennis rackets, golf clubs, speakers, stereo's, etc.
I would recommend you check out my recent Casio PX780 review: Casio PX780 review
As far as the Yamaha YDP181 goes, it's really quite basic (same cabinet & sound system as the V240) and has almost none of the functions of either the Yamaha DGX640 or YDPV240. But it was not built for that as it's main purpose is to give you a good replacement for an upright piano which is what it tries to do and is certainly much better in that way than the other two Yamaha pianos. However the YDP181 does not have a USB ipad/computer output connection on it which is a big drawback in my book. Even the YDPV240 and DGX640 has the USB connector although it is not the new "plug & play" system that the new Casio pianos have which is even better and makes interactive play with an iPad more intuitive. If you have not seen the latest available music & piano educational apps available for the iPad, then you should. They make learning and practicing fun and enjoyable while speeding up the learning process dramatically.
At the end of the day you can be happy with any of these digital pianos because they all do a good job. But some are just better than others in their price range and that's what you need to be aware of when you make a purchase decision. At the rate digital piano technology is going, regular acoustic upright pianos may become a thing of the past one day and it's just a matter of time. But until then, acoustic pianos are still a great investment and can be a very enjoyable playing experience although they have high upkeep costs, are heavy, loud, and cannot be played with headphones, nor can they be connected to laptop computers or iPads. No wonder digital pianos are far outselling acoustic upright pianos these days:)
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