Before I had the Forerunner 305, I had its predecessor, the Garmin Forerunner 301. I really liked the 301, but since I've had the 305, I do not use the 301 anymore.
One thing about the Forerunner 305 is that the expected battery life is only around 10 hours. I go very slowly and my 50 mile competitions can run 12 hours including breaks, so 10 hours is not good. So I got a AA USB battery extender. This is a generic box that is used to charge USB devices using 4 AA batteries. Since the 305 can be charged via the USB port, one of these works well.
I take extra batteries to the vet checks if they're out of camp, but just charge in my truck at in-camp checks. I have a USB plug-in for my car and then just plug my Forerunner 305 cradle into it. Works just fine. I did 2 long 50s (9 hour ride times) last week with my 305 and had no issues with the battery. I did charge during one of the vet checks each day and every night. Never got to charging at the other vet check each day. I've also used the 305 for 5 other long 50s this season and never had a problem with the battery by giving it a boost every little bit. If you have a crew to remember to charge for you at least every couple of loops, you won't
have any trouble getting through a 100.
I'm actually really lucky that my sport has holds where I can recharge very easily. Some other sports are harder to deal with the reduced battery life. This is one thing that disappointed me about the 305, but I really haven't been inconvenienced by it as much as I thought I would.
Now, the thing you might have to think about in a 100 is the memory. I got about 95 miles worth of detailed data on my unit. This is also something you have to think about with the 301, which is also a good unit, but not as good satellite reception and doesn't have some of the workout bells that the 305 has. For me, my hubby downloads the data to his laptop every 2 or 3 loops, so I don't have to worry too much about lost data, but if you don't have a laptop, you'll have to be a little creative, but saving all the detail data can be done. The trackpoint recording thing, though, is very dependent on the trail. The curvier the trail, the less mileage can be stored because the unit is storing more trackpoints to cover the entire trail. If you're going straight for a long time, the unit won't record a trackpoint as often. Garmin calls this "Smart" recording.
I have a 301 and a 305. I can tell you I have not used my 301 since I got my 305. I love my 305. I like the much better reception without having to mess with the external antenna. With the 301, I used mount an external antenna on my helmet, but it's a pain (not to mention more expense) and I don't have to do that at all with my 305. I suspect the 305 is better than the 301 with the external antenna, anyway, but frankly, I don't want to mess with the external antenna stuff anymore and so I don't care to do many tests. I really should, I guess, but...
For those interested in this external antenna solution, here is where I got my battery box. http://www.pc-mobile.net
I got the actual antenna from www.gpsgeek.com. But like I said, I don't use this solution anymore because it's a pain.
I like the 305 because it's much more configurable than the 301. If I want to see one
HUGE data field per screen, I can do that. I can have 1-4 fields per screen.
If I have 1, it's really big.
If I have 2, they take up 1/2 of the screen each.
If I have 3 fields, the top field takes up the top 1/2 of the screen and the other 2 split the remaining half.
And if I have 4 fields, they each take 1/4 of the screen. Even at 1/4 screen, I can see the data just fine. Now, if you have 3 or less screens, the 305 will show the heart rate (in addition to the 1,2, or 3 other fields) as the teeniest, tiniest font you've ever seen!! That takes a magnifying glass and perfect stillness to read! So I don't even count that as a useful thing.
You can see 3 different screens. These screens all can be configured with 1-4 data fields and all the data fields can be configured to whatever you want. You can have the watch stay on one screen and you can switch between the screens by pressing a button. Or you can have the screens cycle automatically for you.
For example, I have the following set up. All screens have 4 fields.
Current Speed (upper left)
Total Time out (upper right)
Total Distance (lower left)
Current Heart Rate (lower right)
Current Lap Time
Current Heart Rate
GPS Accuracy (to be sure I still have a good satellite lock)
Time of Day (to be sure I make the cutoff!)
Lap Average Heart Rate
Current Heart Rate
I have these cycle for me automatically so I get to see everything in turn. I have the heart rate on every screen, so no matter when I look at the watch, I can see the heart rate.
This really helps me with pacing and being careful with my horse through a 50 mile endurance ride.