One of the features that this training log provides is the ability to log workouts on different bikes. So you could have different mileage reports for your road bike and your time trial bike, or for your mountain bike and your road bike.
Another tool that is awesome (and other could learn from) is the sharing scripts that are provided. Yeah I know they don't do social media like Twitter but you can share your latest workouts with a simple script, or even your distance totals. You don't have to update them for every workout they just keep themselves updated as you add more data. This is way better than RunKeeper where you have to grab their iframe code every time you want to share a workout.
So all of these logs let you make maps but Bicycling.com lets you take notes on your maps. Find an awesome little spot to grab a mid-ride snack, just add it. How about the location of the local medical clinic, add it. If you're making your maps public then this information can be invaluable for other cyclists visiting your area.
The first thing about this site is the ads all over the place. If you want to deal with popups regularly (every time you visit) and ads spinning/flashing/growing all over the page then this is where you go. Heck even their forums are littered with ads. I think this is just old media mindset. Rodale (the parent company) has been around for a while and it certainly doesn't seem like they care about their users more than their ads.
While the notes on maps are a pretty killer feature that's about the only killer feature that's good. The other killer feature (as in your dead to me) is that the routes don't snap to roads. Now maybe this app was built in 1999 (and it looks it) when no one had this feature but everyone else has it now. Around me many roads follow rivers so they're all curvy. Entering an accurate route is an ordeal of mythological import.
The fact that maps don't snap to roads would be totally negated (at least for some users) if they let you upload GPS data or had their own app for iPhones/iPods or other mobile device. They don't though so your stuck with their terrible mapping interface. Sorry.
I guess give this one a look if you like lots of flashing annoying ads and a site that's designed like it's 1999. In all honesty this probably fits some people's needs but the fact that the interface is a pain and the map doesn't automatically stick to roads is going to be a deal breaker for most.
My vote is ride on.