I'm not sure how to review this app without including the service, so...
I was thrilled to get my hands on a SIP-configurable WIFI-enabled Nokia handset in the fall of 2006 when I purchased my e61 to replace my old 6600. In fact, at the time of purchase, SIP phone calling was the most anticipated feature for me. I tried COUNTLESS different SIP providers after working around some e61 bugs before a firmware update smoothed things out.
But it was when Nokia launched Gizmo for S60 when I began to use and promote the Gizmo service with everyone I know. To this day however, I don't know how/if Nokia is associated with Gizmo5 (formerly Gizmo Project), but solely because Nokia had built a S60 client presumably in collaboration with Gizmo, I was then biased to give Gizmo an edge over the competition and promote it.
The S60 client's UI got better with a couple revisions and soon I was using the contacts menu integration over Nokia's manual SIP configuration with Internet Telephone Services. However, for comparisons I also installed SIP clients from Vyke, and Truphone, just to name the best of a few alternatives. Today, there are many Symbian and Java based clients to happily handle my SIP accounts, such as Fring, and now Gizmo for S60 has gotten lost in the pack.
Positives-
-Gizmo5 service is full-featured with better than average call quality and reliability. I read a lot about horrible SIP call qualities, specifically with Gizmo, but I have made many hundreds of calls over thousands of minutes with only 5-10% of calls being unacceptable. Most overseas calls are better than GSM quality.
-S60 client integrates seamlessly with my Nokia's native menus.
-IM support for many different services with simple to use bots for configuration. I especially like using the IM feature, but not as much as Palringo, Fring and, now, Nokia Contacts.
-Free voice mail.
-Competitive pricing for the quality, service features and SIP portability.
Negatives-
-I should receive an honorary degree for all the education I've had to provide myself for the understanding of how SIP/VoIP calling actually takes place. Codecs, NAT transversal, Firewall workarounds, port forwarding. To insure Gizmo connects throughout my different WIFI environments/connections, it takes constant troubleshooting and fixes. Most people I recommended Gizmo to have given up on the service.
-Development of Gizmo for S60 seems to have died.
When my Gizmo client is connected and working, I enjoy using it. Answering SIP calls is just like GSM calls and the quality is usually as good or better than GSM, especially for calls between Sweden and the US. And the IM portion of the client is nicely done. However, getting and staying connected is always a challenge as I move from one access point to another.
Bottom line is this: I know there are ways to improve this client because another VoIP/SIP client on my e61 ALWAYS connects and ALWAYS works with no tweaks, fixes, port-forwarding, etc. and it's name is Truphone. Simple, reliable, integration, SMS sending options and connections are automatic and painless. I'm not pitching Truphone, just bringing attention to the fact they seem to have figured out what Gizmo hasn't.
I hung on to Gizmo for a long, long time, pushing it on friends and family, but Truphone has finally taken Gizmo's place for my long distance calling/SMS over that of GSM.