I have one company that uses Google Voice and one that uses Grasshopper. Grasshopper allows you to easily set up a local number and/or an 800 number (you can also import your own number, for instance you GV number). It allows you to setup "lines" (press 1 for support, 2 for sales, etc), which can forward to other numbers, record messages, have on-hold music and a hunt group, etc.

So, to really answer your qustion, I think it depends on what your startup is. If it's a B2C company that really only needs its own number for support issues and whatnot, you're probably good with just a GV account that forwards to your cell phone. If it is a B2B company though that needs to sound established and professional, I'd consider setting up an 800 number with a multiple-line answering service like Grasshopper, even if every line only forwards to your cell phone for now. This also makes it very easy to scale once you start hiring people.