If you put the right equipment in and configure it properly, VoIP can be a great solution. The issue here is that costs a LOT of money and time.
I really try to steer clear of supporting any hosted solution, because I personally have never seen one work 100% as advertised.
I have several clients running anywhere from 200-500 IP stations in multiple site configs that absolutely love the product. they spent a lot of money, probably 3x what it would have cost to install a TDM system. They do save a bit of time and money in the long run because they are not constantly paying for MAC work and can do a majority of their own admin on the system, but most decent TDM system have become pretty user/client friendly over the last few years.

My personal opinion would be to avoid the high cost of VoIP unless you can come up with three reasons you absolutely MUST have it. Most businesses can't.

As far as hosted solutions go, check out Freedom Voice. A pretty decent hosted product with NO CONTRACT. You don't like it, you can bail out with no penalties, strictly a month to month fee per user.

Good Luck!

D