In my experience serviced offices are extremely expensive. You'd want to be making a huge income to pay them. And as for the difference between virtual office and having your own messageminder, email, mobile phone,fax etc, it's not much - do you really need a person to answer calls for you? Is it just a mailing address you need? What about renting space in an Enterprise Centre - they're specifically for start-upps.
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Although in my experience (of two) they only allow one business any given type to use their facilities at any one time so can be difficult to get in to. I've found them doing more to support established businesses they already have than start-ups.
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