I tried this extension (SPTransfer, not SP Upgrade) as a last-ditch effort, and all I can say is WOW. Not only did the extension work great, but because it migrates your settings from one site to another, you don't lose your original site. In other words, no risk.
The help support has been fantastic as well. First time I migrated I encountered one minor anomaly (no menus were transferred), but after emailing the developer I received a response back within hours and discovered that I had made a mistake with one of the settings I used during the transfer. Following the dev's advice, I re-migrated, and everything came over perfectly.
Again - this is a fantastic extension, it works very well, and the support is great. If you've been struggling with an upgrade (or given up on one, as I was about to), then please please PLEASE give this extension a try.
NOTE: SP Upgrade can transfer data between Joomla 1.5 -> 2.5, 1.5 -> 3.0, and 1.5 -> 3.1. SP Transfer (the app I bought and used) can transfer data between a number of combination of Joomla versions (1.6, 1.7, 2.5, 3.0, and 3.1).