If you've ever been on a cardio machine at your local YMCA or fitness studio which has seen its better days, the odds might be against you getting an accurate heart rate reading. Working out at the wrong intensity can be as effective as dieting without portion control; too much (food) can be as bad as too little. I like using heart rate monitors for run training, both for encouraging an athlete to run at an easier pace during training runs when the effort ideally should be lower, as well as for tying a number to a feeling of intensity. Most experienced athletes, I've found, can provide a good ballpark figure of where their heart rate is based on the intensity level of their activity. The heart rate monitor can also help determine whether more recovery time is needed after a hard workout; if the resting heart rate first thing in the morning is as few as five beats higher than normal (depending on the baseline) it might be a signal to forego that early morning jog.
I've played around with, purchased, used, beat the daylights out of, & discarded half a dozen or more different heart rate monitors, including units by some of the major players in the game, namely Nike, Polar, Suunto and Garmin. Depending on the information an athlete needs, their comfort level with technology, brand of computer, willingness or desire to slice-and-dice (export) data, & level of sticker shock they're willing to undergo, there is a model from a maker which may fit nearly every athlete.
One of the first monitors I owned was the Suunto G6, which was pretty much a high-end monitor, with a frightening level of expandability; I purchased an ANT+ compatible footpod which tracked speed & distance by a battery-powered accelerometer. The G6 could do nearly everything imaginable, save for order me a beer at the end of a training run, but it could & would not (at that time) play nicely with my Apple MacBook computer. Since then, Suunto has developed an on-line, fairly user-friendly workout tracking program called MovesCount, but with the MacBook I had to purchase a Windows emulator program to crunch my workout numbers. I had difficulties with the G6 receiver unit's battery life, compounded by minimal U.S.-based technical support (California, if I rightly recall...). Since that time, Suunto has expanded its presence in the U.S., especially in the major metropolitan areas, & may still have a package deal with Carmichael Training Systems, a company owned by Lance Armstrong's former coach Chris Carmichael.
Most good-quality HR monitors will tell you how long you were exercising in a particular target heart rate range. Others will tell you how long you spent in particular heart rate zone. If you want to know whether your workout was effective or if you're on the edge of overtraining, again, look to Suunto monitors. The monitor & accompanying software programs will show you progress over time and the training effect of the workout you completed - measured by Exercise (Post) Oxygen Consumption (EPOC). EPOC measures the "calorie burn" which persists after the workout is completed...kind of like measuring how long the hood of your car will scald your bare legs should you decide to sit on it in shorts after driving all day.
If you decide to keep a HR monitor for a long period of time (read: send it in for repairs when it breaks or the batteries die) I still consider products by Nike & Garmin to be the best bet, since they have regional service centers. When I owned the Suunto there was only one place - San Diego, I think - to send the monitor for TLC.
Nike's C8 monitor had software & components which were easy to use & fairly user-friendly, with on-line "set-and-forget" (or "set-and-regret") functionality. I could also use the strap on spin bikes & Life Fitness equipment at the local YMCA; I never thought there was a downside to having a non-coded transmitter strap, but my wife was always a little less-than-enthused when her elliptical trainer workout would suddenly change intensity...it would pick up my heart rate & adjust accordingly. But...get that thing near any amount of water & you could guarantee a bad day...a guy who sweats a great deal should never use electronics which are not waterproof.
My most recent acquisition is the Garmin Forerunner 310 XT, which is a GPS unit with ANT+ compatible footpod for treadmill running, ANT+ wireless cadence meter for power estimation on the bicycle, & has, in my humble opinion, the most comfortable HR strap I've used. Oh, & I guess I should mention since it's built specifically for triathlon, it's waterproof. It has a battery life of about 20 hours, but how many people wear their HR receiver the entire day?
My wife will use gymnasium cardio machines, & keeps track of how hard she works by the pulse pads, but she absolutely hates the thought of having anything rub her chest or wrist...it irritates her (She rarely wears her ring, but we don't mind...) to no end. Even the Mio, which uses two finger pads on the monitor, was out of the question...definitely no good for cycling. I almost had her talked into trying out a Polar HR system a couple of years ago; it seems as though enough women complained about how difficult it was to wear a sports brassiere AND a heart rate monitor strap. The fine folks at adidas developed a running top which provided a modicum of support & a means for attaching the chest strap. But, as my wife likes to remind me, support wear is a personal thing, & one size does not fit all...it only fits one size.
Back to Polar, I've seen mostly their more inexpensive HR monitors, which tell the HR. Since my spin class had a bunch of these & my most recent HR monitor is not Polar compatible, I ended up using their strap. Functional, simple. Friends of mine have higher-end units: Polar is good at providing products with incremental increases in quality, features (and price). Their highest-end HR set-ups will tell you nearly everything you would need to know, work with programs like TrainingPeaks, & can help the individual athlete set up training plans.
But, if you're not going to figure out what that flashing number means you only have a couple more ounces of plastic, glass & gadgetry strapped to your body. Like I said before, the data is only as good as what you do with it, & what decisions you make because of it.