Yes, Iâ€™m using it (always extending the page type, but nothing fancy). I donâ€™t remember having problems when updating. Your error is not very descriptive but, in any case, itâ€™s complaining about the name of a required table in the database.
Had a very similar problem when I backed up a live site to a dev machine to test an upgrade to 2.3.7 and to add some additional modules. The upgrade went OK as did adding the extra modules.
The problem arose when I tried to upgrade userforms to v0.2.1 from v0.1.0
The Manifest was not getting updated properly - the cached details about the classes added or changed by the updated version. This appears to be caused by the way the ManifestBuilder determines which cache files are updated when the dev/build is performed.
Since the timestamps on the files unpacked from userforms-v0.2.1.tar.gz were prior to the date of the dev/build which cached the details for userforms-v0.1.0, the cache was not updated correctly. Deleting the cache files from silverstripe-cache/ and re-performing dev/build cured it.
"UserDefinedForm_EmailRecipient" is a new class added by v0.2.0 and can't be found in $_ALL_CLASSES['parents'][$class] because it is not (yet) in the Manifest (cache).