The GPS you buy depends on your needs. One of the key differences is GPSs that have the ability to turn off auto route recalculation. A typical Garmin NUVI does not have this ability while a Garmin 78, more appropriate for off-road riding does. If I understand correctly, the Montana also has the ability to turn off auto route recalculation.
Why is this feature so important? If you spend hours designing the perfect trip on BaseCamp, selecting your favorite roads and scenic byways, and then send this route to a Garmin Nuvi, it will recalculate your route selecting the roads it seems more suited based on your preferences entered in setup (avoid tolls, avoid highways, etc...).
The correct way to do this with a Nuvi using BaseCamp is to create a trip using trip planner and enter enough way (via or shaping) points that the Nuvi has no option but to follow the route you have programmed. Once you understand what's happening, it's no big deal.
When riding off-road, you need a GPS that will follow at GPX file exactly and not perform any route recalculations. NUVIs can't do this. Montanas and Garmin 78s can.
If you ride on-road then, a NUVI will be fine. Off-road and you'll want a GPS that accepts GPX files with the ability to turn off route recalculation. Since this is a Honda ST forum, I'm guessing the NUVI is what you would be happy with. I ride a Yamaha Super Tenere so I needed off-road capability. I have both a NUVI and a Garmin 78 on my bike.
My best suggestion for a Honda ST rider is one of the Garmin NUVI 25xx series.
http://amzn.to/1wXPCH8
The 2595 has Bluetooth which will connect to your helmet headset (ex. Sena) and phone. In all the models (LMT) stands for lifetime maps. Garmin makes more expensive units with higher resolution screens but these have disadvantages. The 5" screen the 25xx models have is low resolution which means the letters and numbers are bigger. With my old eyes, that means they are easier to read.
As for Bluetooth, I hate having that voice talking to me all day. I connect my Sena unit to my phone to listen to music and make phone calls so I never connect to my Nuvi. So, don't get hung up on Bluetooth unless you're sure you'll really need it on the GPS.
As for waterproofing your $100 NUVI vs spending $500+ on a unit that is waterproof... Yes, I saw the Youtube video, used silicone on the edges of the screen and did actually hose down my GPS (forgot it was on there...) and it works fine. I ride in the rain all the time in Oregon and don't worry about it.
One other downside to the more expensive (Montanas) is that they don't have the most current (user friendly) software that the newer NUVIs do. I'm pretty happy with my 2955LMT and Garmin 78 combo. As a plus, my 78 has marine maps that I can use on my boat.
Don't forget to budget for Ram mounts and to protect from sun glare, get a glare stomper.
http://amzn.to/14ZeC9s
Youtube video is here:
https://www.youtube.com/watch?v=s2UvIoL78Cg