The situation is a bit more complicated these days than "mtgox still leading" or "gox is dead".
In the past weeks, several rallies were lead by exchanges other than gox. On most days, gox volume is actually lower than that on bitstamp, for example. But once in a while, like now, when a big move starts, gox is still at the center of attention: they do have the market depth (and deep pockets and experience of traders on the exchange) that no other exchange matches so far.
See what I mean? Gox is almost certainly not the only relevant exchange these days. Ignoring what bitstamp or btc china does will cost you money. But on the other hand, gox is still in a unique position, and it is not wrong to say that, more often than not, you're well advised to watch what is going on there.
One more remark: I got the impression that, in the recent past, gox did in fact lead several *rallies*, but bitstamp was quite successful in *stopping* them (i.e. when gox price went too far ahead, and bitstamp was only reluctant to follow, it went down again on gox)
Forgive me for quoting myself. But here's a nice example of the effect I described above:
Mtgox reaches today's high at 15:35. Bitstamp takes another hour before it reaches its own daily high, at 16:45.
Ergo, mtgox still "leads".
Same time, 16:45, bitstamp iniates downwards correction, 225.5 --> 224, then 223. At this point, mtgox trades sideways. Takes another ~15 min before the correction gathers speed on mtgox as well.
Ergo, bitstamp "leads" as well.
