I've used entry level Naim gear (both 3 series CD players and the Nait 3 integrated) for about 6 years. As long as the recordings are good fatigue has not been a problem, even during "all day" sessions. Naim gear will not rescue you from a bad recording (what will?), but it seems to be particularly good for getting the most out of older recordings, which is a boon for the classical fan.
I've found Naim gear to have almost a tube-like sound. The CD3 in particular had a dark, rich midrange which I loved (alas, mine had an irreparable transport failure). The CD 3.5 and the 5 series have gotten a little brighter and better at ambience, apparently at the expense of the midrange, although I found that adding a flat cap brought back the midrange magic in the CD 3.5. I don't have experience with Naim's higher level gear, but the Stereophile reivews indicate that the entry level CD players may be less fatiguing and more forgiving than the CDX. To sum up, I don't think you have to fear undue fatigue with Naim's entry level gear, whether the 5 series or the 3 series (excellent value on the used market).
Addressing the other half of Budrew's original question, Naim gear is wonderful for classical music. It may lack the nth degree of soundstage and detail, but it's certainly good enough in those respects, and the sound is rich and engaging -- evident even to non-audiophiles. I once heard the CD3 in an a/b comparison with the Rega Planet playing various acousitc chamber music from analogue masters. The Naim made the Rega (which is not a bad player) sound like nails on a chalk board. I also recently heard the 5 series edge out (albeit slightly) a much more expensive Linn system on a recent orchestral recording. If anything, I'd say Naim is a little weaker on rock music than on classical/acoustic since it lacks the slam of, say, Krell equipment.
Naim gear tends to inspire extreme views -- people either love it or hate it -- and there are certainly viable alternatives. I've found the entry level gear to be very satisfying (and I'm kind of picky), good all around performers, and good value for the money compared to competitors. You may like something else better but Naim is a viable choice.
