In my last job I used it extensively talking to one of our manufacturers in China (for the excuse/s of the day)! Worked extremely well both using a handset or a PC mic and speakers.
Its easy to set up and use. Only concern is that one or two colleagues started to get some odd unsolicited calls such as from a guy in Israel who "wanted to practice his English" (I kid you not)!. Though these can be screened out.
Not entirely sure but I was in the office on two or more occasions when a colleague PC's rung to indicate that someone was calling.
That said it was nothing sinister and such things can be screened out.
Don't let that put you off(for business or personal reasons) from trying this or similar software if you are in a position to make PC to PC calls since it works well and will undoubtedly save you money