I have heard variable things about Vonage and I am sure there was some question about legal issues surrounding their service.
As for softphones...just don't do it..not as a primary phone anyway, buy a proper SIP desk or DECT phone. On both Linux and Windows I have had very variable experiences with practically every softphone I tried.
I keep one on my (windows) laptop for emergency use if I am away from either my house or the office (where there are Aastra hardware phones available)
For desk (wired) phones I highly recommend Aastra and suggest you order them from Digium.co.uk the Aastra phones (I only have the cheapest ones) are extremely well built, have a simple control layout and an excellent speakerphone. For DECT I have used SIP enabled Siemens Gigasets but if you are looking at asterisk they don't have all the facilities like quick transfer, swap etc that you might want to get the best out of the asterisk box
SIP and NAT is a total pain as Brett has suggested. I have managed to get a SIP phone at home connected to our office Asterisk server when both are behind NAT's but I suspect if I wanted more than one phone at a particular destination I would have to put the Asterisk box in a DMZ.
On a related note, I have a BT Voyager 220V ADSL/VOIP router here that is surplus to requirements, it needs a 12VDC wall wart capable of 1.2A and currently it is locked down to BT services only (for both the ADSL and SIP services) but I also have a copy of some hacked firmware that is supposed to remove these restrictions (untested as I have no need for it now)..Free to anyone who can be bothered to pick it up or is attending the next Ipswich meet