How accurate are the calorie displays on equipment?!

I wouldn't trust the machines. I've read that they over calculate by 20% at times, however I don't necessarily trust mfp either. A heart rate monitor is your best bet but there are some great calculators online. I use the one on healthstatus.com

Get a HRM, because the machines vary depending on who they are calculated for. Some times the machine is way over, some times way under and some times it matches my HRM pretty closely. You can't really rely on that information. HRM's are about $60 and worth every penny.
In my highest burning zone (80% intensity) I burn 10 calories per minute.

Depends on the machine... some are as accurate as you can get, others... well, not so much.

Not at all. Get a HRM.

there is no guarantee that an HRM will be any more accurate. Ultimately it comes down to the formula they use to ESTIMATE calories burned.

Find the average. Compare the machines with various other means. HRM + Calorie burn Calculator for example. see where they all compare and you'll figure out an average you can then use the machine wisely and adjust by the difference you know it is roughly off by then play it smart and under cut an extra 5% to 15% from the burn and use that total. Sure you cut yourself short on how many you truly burned but it's better to under cut your total then over cut.

Get a HRM, because the machines vary depending on who they are calculated for. Some times the machine is way over, some times way under and some times it matches my HRM pretty closely. You can't really rely on that information. HRM's are about $60 and worth every penny.
In my highest burning zone (80% intensity) I burn 10 calories per minute.

We are pretty similar. I burn around 100 calories a mile and my "just going for a run" time is 10 minute miles (6mph), so that's roughly 600 calories an hour just like you. If I get into intense working out, I can get 700+ an hr.

Get a HRM, because the machines vary depending on who they are calculated for. Some times the machine is way over, some times way under and some times it matches my HRM pretty closely. You can't really rely on that information. HRM's are about $60 and worth every penny.
In my highest burning zone (80% intensity) I burn 10 calories per minute.

Have to agree with this. I wore mine for several sessions of all the cardio I do over the course of several weeks: spinning, hiking, circuit, running and walking uphill. ALL (minus the hiking since that's outside) were lower than what the machine says. Afterwards I use those numbers/times as part of my mfp cardio. If your paranoid, take another 5% off each number and base your entry on that.

I want to know how accurate the displays are... considering that I'm going to burn more calories sweating like a pig compared to the 120 pound woman next to me who hasn't broken a sweat in the last 30 minutes.
These displays don't take in consideration the weight/effort of the person on them.... should I log those calories or the ones listed in MFP?!?! HELP!!!! I want to be more accurate!

Some people sweat at the drop of a hat .... not a good indicator.

Calorie burns depend upon: height, weight, age, gender, and exertion level (among other things) ...... how much information do you plug into the machine?

If the machine does not know your gender (it "assumes") ..... men burn more calories than women because they have more muscle mass. So if the machine assumed male & you're not .... it's wrong.

Exertion level - this is not just jogging at X MPH on XX% incline ...... for some that would be a killer workout .... and an easy on for others.

MFP may not be any better. Yes - they have some of the information ..... but are all exercises based upon all available data?

A heart rate monitor "should" be more accurate (not everyone agrees). A HRM with a chest strap attempts to measure your exertion level by comparing your resting heart rate to a continuous reading while you are workoing out.

I have a Polar F4 (Amazon) ..... I really like it.

Not at all. Get a HRM.

there is no guarantee that an HRM will be any more accurate. Ultimately it comes down to the formula they use to ESTIMATE calories burned.

HRMs use user input data, so it is FAR more accurate than machines.

Not necessarily. Doesn't matter what info you give it... a crappy formula is still going to do a crappy job estimating.