I have used domains with numbers (in my private / hobby life), and people have consistently responded negatively to them. Also, they're pretty hard to read out loud to someone over the telephone, most people are not used to mixing letters and numbers in domains.
Having said that, if the number is part of an overall branding strategy that ties to things people recognize from off-line life, as in Gary E's excellent example, then I guess they can work very well. But "43Folders" and "37Signals" are just hard to remember and read to someone, without any significant benefit.
Finding free .com domains for new companies is getting quite hard, so I understand the temptation to name a company or service something with a number. But I think it's better to use a prefix or suffix that has some meaning to the target audience. Examples could be "software", "net", "cloud" (only if specific to cloud computing); geographical names (city / state / region), etc.
As always, one should avoid naming a company or product in the currently fashionable style without good reason. For example, above I mentioned "cloud", but if cloud computing turns out to be a fad that burns out, then having "cloud" in your company name can send all the wrong signals real fast.