I purchased the grey, which actually has a suede upper rather than leather.  I've worn the 621 and 622 and liked them both.  While still a decent shoe, the 623 is not as good.  Here's what I don't like compared to the other models:  the suede upper does not give as much lateral support - in fact, I have to keep tightening the shoe strings throughout my workout; the shoe runs a little larger than the other models; but most of all, the forefront cushioning is not very good at all.  Now, I use my shoes for working out and cross-training, so if you just want a walking/work shoe, this should be fine; just doesn't do that well with heavy pounding.  If you get the 623 for exercise, opt for the one with a leather upper - probably better.  However, one plus for the suede is that it is cooler and more breathable than the leather, so if you wear all day, something to consider.