Chuck sed--
Taken to extremes, the limit with Newtonians anyway, is when the shadow of the secondary mirror gets large enough to completely fill your pupil- then you see nothing at all, on-axis!
That's the problem I have when the exit pupil gets too large for my pupil. The secondary takes a larger and larger portion of the image reaching the eye, and it can be quite difficult to hold an image. For me anyway, long eye-relief makes it even trickier, since you have to keep your head in a perfect position. Even though there might be higher resolution in the image, it's hard for me to deal with that odd blind spot bobbing around out there. Just goes to show you how personal the match may be between observer and equipment. I've often gotten a so-so view with an eyepiece someone else is raving about--even when it's obvious that eyepiece is really showing them a lot. This is why you have to be cautious about recommendations.