Ok, here's a con to doing observed tests. I personally think they are a lot closer than a formula. I would also recommend that if you decide to do a climbing test, that you not do it alone. It isn't likely that you can have a problem if you've been exercising regularly. From what I've read about it, you will get to a pain zone before you injure your heart. Few people can "push" beyond that level. However, you will be on a hill and presumably clipped into the pedals and exhausted. A liittle support isn't a bad idea.
Nevertheless, there's a flaw in anchoring your workout to your maximum heart rate. Few people know what it actually is.