I used to work at Aptela, which has a nifty, robust, and fairly affordable Web-based PBX product. I've used it at 3 startups and have been happy with the performance.
On the other hand, they also have a VoIP offering that is still unstable and that most customers still won't recommend. To be fair, this is primarily due to the fact that there is still no reliable VoIP backbone vendor for the SMB market. Regardless, does anyone have any other suggestions?
VOIP is unstable past about 12 lines unless you have a lot of network infrastructure.
VOIP is good for a 3-8 person operation or as an add on to a regular PBX (for work at home or work on the road).
I use PBNext myself (mostly for the 866 number) but I wouldn't try to run an operation of more than 8 people on it unless I was in the same building as my ISP.
I'm a month in using phone.com, and I have to say I am very impressed. While it takes a little bit of work to get comfortable configuring it, and there are a couple of minor quirks, it is hands down the best soft PBX I've ever used.