I use a Roland RD-700 digital piano, and the action is absolutely amazing. To me, it feels almost exactly like the Yamaha grand I play on at my lesson. The weight and feel of the keys are absolutely not an issue. Absolutely a good digital will feel and probably sound better than an old upright.
Now, the sound of a digital is another story all together.. My Roland does a pretty good job, I am 95% happy, though I play it through good studio monitors in an acoustically treated room. The samples are all high quality, and you do get a certain degree of the overtones ringing.. Sounds very good in fact and for 95% of the work I'm doing on it (learning repertoir, basic technique work) it's actually quite suitable.
But as of late I'm noticing that much of the detailed technique/articulation work I do with my teacher on the acoustic grand I just can't quite translate.. The "feel" of an acoustic where you can actually "pull sound out" of the strings, and "feathering" the pedal etc. etc... Even advanced velocity sensitivity in a digital can only come close, in the sound produced.. but it's not there.. and then there's the issue of sound coming from two speakers, rather than enveloping you from all directions as your body vibrates as well!
which is why inspite of the fact that I have a high-end digital, which is great for practice, great for portability, great for headphones late at night, great for all the other instrument samples it's got for compositional purposes, I am beginning the consideration process for an actual grand.
-Paul