I carry personal 406 EPIRB for emergencies and a SPOT Messenger which I think is a great peice of kit - but it serves a different purpose.
As I build up my solo miles I use the SPOT 'OK' Messenger to keep my family and parents up to date with when I'm setting off and progress (I don't tell them where I am necessarily heading or by when I will arrive, I would log a passage plan with the coastguard if I felt it necessary on a longer trip).... and it reassures them all.
I cannot see myself using either the 'Help' or 'Emergency/911'button in the UK as if I really needed help (after all self rescue has failed !) I would use ...mobile phone, hand-held VHF, main VHF/DSC set, EPIRB and flares/stobe etc first ?
So in summary I don't consider the SPOT as a 'safety' device ... just a tracker for providing updates.
Personally it also has an added benefit that I like taking photographs along the way and there is a great facility linked to the SPOT which allows you to document your track, waypoints etc.; an example from an earlier trip is shown below:
http://www.spotadventures.com/trip/view?trip_id=169621
Plus I also use it in a smiliar way for tracking mountain bike trips.
Hope this helps.
Cheers, Andrew