I've been using Skype since it was introduced. Works well enough for what I'm paying (near zilch). It's been fairly reliable, but does seem to attract an amazing number of VoIP spammers. My major use is not for calling in or out of the PSTN. It's Skype-to-Skype calls to walk customers through computah problems. Another use is to provide the voice part of remote control ham radio stations:
Running Skype on my Windoze Mobile 5 cell phone was no fun. It worked but was clumsy and not very reliable (using Wi-Fi). The software would also crash erratically. My guess is lack of sufficient horsepower in my Verizon XV6700:
However, I haven't tried any updates for about 6 months, and there may have been some progress.
For cheapo phone service, I prefer something that will do G.711 and G.729 when necessary. Also, something that will also operate with a fairly standard SIP Phone. For that, I use GizmoProject:
However, most of the time, my messaging is handled with either some form of IM (instant messaging) or cellular SMS (short message service). Trying to walk someone that can't listen, type, or think, through a problem is tricky over the phone. I do better by using IM to send them URL's, command line incantations, and reduced size screen dumps. Favorits are Skype Chat (built into Skype) and Pidgin (multi-vendor IM client):
So, what problem are you trying to solve with Skype?