I recently started using VoIP. My first experience was with Ekiga using a cheap handset, which I bought at Conrad for 20 Euros. I choose DiamondCard.us as my VoIP gateway, it happened to be directly supported by Ekiga.
The quality of the calls made using the VoIP gateway was lower than that which I was used to over a normal telephone line, however, it was acceptable. The reason for the quality degradation was difficult to diagnose given the large number of variables (handset, connection, provider) so I did not investigate very much.
Isabel then changed telephone and Internet providers--from T-Com to Alice. Although Alice is cheaper, the real deciding factor was the lack of a contract: it is possible to cancel the connection with 4 weeks of notice; to get a good deal with T-Com, you have to sign a 24 month contract.
One of the disadvantages of the Alice is that it is no longer possible to use the cheap prefixes (e.g., those at http://www.billiger-telefonieren.de/). Since I call the US a fair amount, it was pretty important to find some solution. We decided to invest in a Linksys SPA-3102, which allows the transparent use of VoIP over a normal telephone handset in conjunction with a traditional POTS connection.
The Linksys come recommended, but with the caveat that it was hard to set up. The solution that I ended up using was to route all calls over the VoIP gateway: in our case, this turns out to be cheaper for every type of call. I do not want to say that this was trivial to set up, however, it was really quite easy. Developing even a simple dial plan (to route calls or rewrite numbers), however, looks like quite a project.
The quality is really excellent and I conclude that my initial poor experiences were due to the cheap handset. I can highly recommend a similar set up.