There are some heavy duty gotchas so this is what i did to get it working.
Make sure you're at J1.5.26. Use Akeeba backup (3.4.3 - not higher) to make a copy of the site. Set up a test server, create a new database and restore a copy of the site. During the restore (i used akeeba kickstart) make a note are you using mysql or mysqli (probably mysqli).
On the test server, rename your htaccess file to htaccess.txt and create a folder on the root called jupgrade. Inside that folder extract joomla 2.5.4 (not higher, jupgrade won't work otherwise). Doing it this way means you don't have problems with curl and its just faster. Log in to the test site and under config > site, uncheck use Apache module_rewrite if its on. Under plugins > system enable MooTools Upgrade. Now install JUpgrade 2.5.2 and in the Parameters menu, change skip checks, skip download and skip decompress to YES. Note that jupgrade uses the mysqli driver by default so if you chose mysql during the akeeba restore you will need to change that here also. Save those changes and click on the upgrade button.
If it fails/stalls, you can try again but remember to dump out any j25_ tables in the database, clear out the jupgrade folder and start over. If it works, it does it in a couple of minutes. Your original site is preserved, your new site is in the jupgrade folder - so you have 2 different paths for each. But both are there for reference and you can start to fix stuff til its right.