I love what my 500 does. The cadence/speed sensor is being a bit finicky right now, but I think it's my fault. It moved on me and I ended up moving it more because the speed sensor was double registering the magnet, and when I tweaked it to make it work the cadence magnet started double registering. Need to slide it farther back along the chain stay a bit.
Don't bother getting a package with the HRM. I like the cadence/speed sensor (when I haven't messed it up), but the HRM straps have been nothing but a problem, and that included a warrantee replacement to the new style. The monitor itself is great, but the strap doesn't last that long. If you want heart rate monitor functionality get a competitor's ANT+ transmitter.
Battery life on the 500 is really good, and they're decently priced at the moment.