I recently bought a Holux M-215+ GPS puck
and wanted to share my expereince. I actually bought it as a backup to the BU-353-S4 as I didn't want a single
point of failure.
It will now become my primary GPS
device for my laptops and tablets. The reception
and startup speed is amazing. Although I haven't used it on a boat yet, it locks onto my location even sitting inside my house (not near a window). Its also seems to get a lock within 5 seconds of plugging it in.
I believe the performance is due to the fact it uses both GPS and GLONASS satelites, effectively doubling the number of satelites it can lock onto.
Anyway.. For $40 its definately a good investment even if you just keep it as a backup. Here is the link to it on Amazon.