There's a thread at SeqAnswers that asks whether researchers will stop using the Roche 454 as the Illumina chemistry improves. While the answers vary, many contributors to the thread note that Roche "won't stand still," as Maubp puts it.
Mike the Mad Biologist gives his take on his own blog, where he says that he has already stopped using 454 for his microbial genomics work. "Compared to Illumina assemblies that use 5 kb jumps (and are thus able to span most of the repetitive regions), 454 isn't even in the same league," Mike says. "Combined with the much lower cost of generating Illumina sequence data, 454 is really out of the bacterial genome game." However, Mike adds that "I could see 454 being useful if you need long reads and not that many of them."