Myfitnesspal accurate?

It really depends on the person and the activity. For me, the calories were usually pretty close to what my HRM gave me. But it does have a tendency to give you too many calories for some activities (I'm thinking of things like the elliptical or zumba).

You can judge based on your own results, though. If you lose at the pace you expect, then great. But if you're losing slower than you expect then trying cutting back to eating 50-75% of those earned calories.

For database items that actually have a level of effort, and you really did that the whole time you've entered - yes.

Like did you really walk 3.5 mph level for 45 min?
Or did you start at 2.5, working your way up slowly to 3.5 half-way through?
That would be 3 mph for 22 min, 3.5 for 23 min say, probably not a huge difference, but you get the idea if you were say running.
Or say you ran outside for 60 min, distance unknown. Then how could you possibly pick the right entry. And if you did inclines, then even if distance is known and you can get avg speed, likely need to round up since inclines is more effort.

Spin bike, no description, but I've noticed that a 45 min Spin class usually has 5 min warm-up much slower, and 5-10 min cool-down slower with stretching.
So class may have been 45 min, but actual use of Spin bike at normal intensity was only 30 min, and should be logged as such.

As mentioned, the cheaper HRM's, even Polar, make some big assumptions to calculate calorie burn, combined with using the HRM with workouts the calorie burn formula isn't even correct usage with, make it rather iffy.
If doing huge amounts of endurance cardio for training and needing to get more accurate , sure that would be useful. If doing 3-4 hrs weekly, not that big a deal.

Thank goodness people ahve posted the responses they have. Its popular to slate the estimates that MFP gives. They can be out for many things, but not everything. As dianethegeek says test by results.

People often exaggerate their own effort, so they pick a more generous category. The point that Heybales makes (thanks).

The rower from what ive read tends to be a reasonable estimation and you can go to the Concept2 site for an adjustment to take into account your weight. Dont worry too much about HRM its just a tool and you can survive without one. Listen to your body and make a note of your results so you know whether you are getting faster or going further.

If you are worried about accuracy then decide whether you want to eatback 0-75% of calories as protection from eating more than you have burned.