Based on the latest data, Comet ISON will not come anywhere near its predicted “best comet in 400 years” status. If fact it may not be visible to the unaided at all. No one is more disappointed than I.
The lasted data (Aug 12, 2013) indicates the comet is five times dimmer than predicted. Sadly, the comet has not brightened since the last observations in May, just before the comet went out of view behind the Sun. Just why it is this dim is not well understood. At ISON’s current distance from the Sun, ices should start turning to gas (cooking off) which in turn make the comet appear brighter.
One possible explanation is the comet is a lot smaller than originally thought. Other explanations have to do with the nature of comets on their first pass by the Sun of which ISON is one. Such comets are typically slow to brighten and never get as bright as hoped as with the most recent comet, PANNSTARRS. ISON is different in one important respect. It will come much closer to the Sun than most comets. We have never observed a first time comet come so close.
It is still too early to predict what we will end up seeing. The current data indicates ISON could be fairly bright easy to find comet to something that will require darks skies and binoculars to see. In any event predictions of this comet being as bright as a full moon are now very unlikely.
If you wish to follow the comet’s progress in detail I suggest going here. Let’s hope the data over the next month or so shows the comet significantly brightening.
Wishing you clear skies