From experience. The altimeter and compass in the "S" models are virtually worthless. They don't cost much extra it's true, but the compass is a battery sucker. Here's why I don't like them:

The only advantage the S compass has is that it works when you're not moving. Big deal, most people use a GPS when they're moving, and it doesn't take much movement to use the regular "compass". Certainly everybody on this forum will have a magnetic compass anyway, for backup at the least.

The S altimeter. It is very apparent from the Garmin GPS forums that the majority of people have no idea how to use them properly. They seem to miss the point that it's really measuring local barometric pressure. As the weather changes, or as you move location, the barometric pressure can change, and so will your displayed "altitude". I repeat: you need to recalibrate regularly, if the weather or your location changes more than a bit! Most people rarely even calibrate, and if they do, may not have a local pressure to calibrate to (typically people get readings taken at an airport, which is not much use unless you're there).

I suggest reading the Kestrel 4000 manual for a better explanation than what Garmin gives. I think Garmin doesn't really want you to know just how inaccurate that "altimeter" is in most situations for the average person. You're better off with the GPS-determined altitude in most cases, especially if you have SiRF not much excuse not to go by that, you'll have enough sats. It's mindless, and way more accurate for almost everybody. If you have the discipline of a pilot, then maybe the S altimeter is for you.

I got sucked in too. But it will be no S next time. Rant over! Just trying to save you a few $$

You're completely wrong about the digital compass. When you want to see where to go on your GPS map you will usually stop to take a look at you GPS and take a new bearing on a land object (except you're riding some vehicle). The orientation of your GPS map will be lost at once, or you'll have to know exactly from which direction you came to orient it properly. Or to walk at least 10 yards in some straight direction to make a new one (usually much more than 10!). Not convenient in many situations. The backup compass is a backup compass. I doubt you'll be holding them both in your hands when orienting on the trail.

^ I guess I use my GPS differently. I have not turned on my S compass for at least a couple years. But you are right, it's just my opinion that it's a useless battery-sucking gimmick. Many people would say the whole GPS unit is just that... I'm fairly flexible. Also, I suppose many people might like it on the water. I still prefer a "manual" magnetic compass, it is not a backup for me.

GPS should be quite accurate for small, short, changes of position. Most of the error terms vary relatively slowly. 1 yard is plenty in my experience. For me orientation has always been a non-issue.

I do think it helps to set the GPS to "north up" rather than "track up". With "track up" the displayed map keeps rotating with your direction of travel, and that makes it harder to relate to paper maps and the real world and to keep it related. With "north up", if the unit shows you moving left to right you know you are heading west in the real world.

Wouldn't "moving left to right" be heading east? The old Boy Scout saying I learned explained the directions going clockwise from the top: Never Eat Shredded Wheat (North, East, South, West).

Oops, yeah.

Lucky you I rarely have the GPS location precision better than 5 meters (yards) while hiking in the woods with any of my GPSes. Maybe you're confused with the car navigation devices and software? They have a special algorithm which adjusts your position to the road, so its readings looks stable, continuous, and with very small error. IMHO, that's a plain profanation of reliable navigation principles.

Ditto "LUCKY YOU".

In my experience, one (1) yard accuracy is just out of the question.