I've purchased several calipers over the years, with the most recent (and highest end) set arriving this week.  That got me to thinking about their relative strengths for my uses and decided to share some thoughts with this community.

In the photo below are the four calipers I currently own - all have a 6 inch / 150mm range. From left to right, are:

• General branded, Swiss-made, synthetic.  These have a resolution of .001 (one-thousandth) inch and weigh 1.5 ounces (43 grams). The right side of the main scale has a secondary, metric scale with a 1mm resolution.  
• Frankfort branded, Chinese-made stainless.  These resolve to .001 inch and weigh 6.9 ounces (196 g).  There are no secondary metric markings.
• General branded, USA made synthetic.  These resolve to .01 inch, weigh 1.3 ounces (37 g), and have secondary metric markings (1mm resolution) and a secondary scale on the dial showing fractional inches.
• Mitutoyo stainless steel with digital readout.  Resolve to .0005 inches, .01 mm.  They weigh 5.6 ounces (159g) and measure imperial and metric equally happily.

Some general learnings

• Plastic is not necessarily an indicator of "cheap".  I purchased the yellow General calipers in 1978 and to this day are still accurate enough for 90% of the measurements I make.  Even the action is decent, though minor adjustments in the thousandths are tricky due to a very slight bind in the mechanism.  Some consideration has to be given for jaw flex - i.e., inconsistent use of the thumbwheel can lead to inconsistent measurements.
• Chinese made is also not an indicator (by itself) of "cheap".  I purchased the Franklins in the mid-'90s and they became the go-to tool for some light machining tasks.  The action is very smooth and measurements are reliably repeatable.  
• Digital is not better than dial.  The Franklin dial and Mitutoyo digitals are very consistent one to the other. By that, I mean one consistently measures> .001 longer than the other from one to four inches. I personally prefer the dial for the sheer mechanical, fine watch aspect, and it doesn't require batteries.  Digital has the advantage of being completely agnostic about units of measure (imperial or metric measurement). Pick your poison.
  • Alas, all devices are not created equal; a cheapie digital set I picked up at a "tool man" type display was so bad I set them down somewhere - I don't know where - and don't care to ever find them again. Inconsistent and awful action - but if I'd bought these first, I'd likely have been ok with them for simple tasks
• Plastic is better than metal - for some things.  Weight and relative softness are the keys here.  I find it much easier to use the yellow Generals for model building - they balance better and there's no danger of scratching a finish or crushing a delicate model if it slips from your grasp (I'm looking at you, Tichy).
• Metal is better than plastic - for some other things.  If you need to scribe or need measurement consistency, you're more likely to find it here.
• More resolution is not necessarily better than less.  For 90% of what I measure, I need a .01 accuracy.  For another 5%, .1 inch is all that I need.  For the rest, I may want .001.  For sorting brass wire, for example, they all work just fine.
• Longer isn't necessarily better than shorter.  Most calipers on the mainstream market today have a 6 inch / 150mm measuring range.  I can count on two hands the number of times I've used anything beyond four inches.  Shorter calipers will be lighter and are less likely to get in your way (or caught on a shirt sleeve) on the workbench.  Alas, shorter ones are less common and sometimes more expensive.
• Branded are not necessarily better than unbranded. My private-label branded dial calipers are of very high quality; see the point on digital vs. dial above.  However, if buying unbranded, I'd advise buying from a source known to offer high-quality unbranded specimens (see the last paragraph).  If buying from an unknown source (eBay, Amazon), shopping by brand is a safe proxy for quality.
• You get what you pay for.  This is generally true on the high-precision end, but it's up to the end user to decide what level of precision is needed.  

Following are back-lit photos of the closed jaws of my four calipers that serve as a good indicator of the caliper's relative quality.  I've also included some comments specific to each:

Yellow General (Swiss).  The jaw fit is not perfect, but it is very serviceable!  I've had these for thirty-eight years and they are by far my most used set.  They were purchased for a different hobby - and while still appropriate for that one, they are used far more these days in modeling.  They are also my go-to device when measuring depth in narrow spaces (i.e, a hole) - the depth gauge is a round rod about .05" in diameter, so it can go places that others (that have a narrow rectangular cross section) can't.

Orange General (USA) - These have a significant gap at the tip of the jaws and their action is mediocre, but they are still serviceable depending on what you want to use them for.  I generally use these only to fractional bolt diameters when I don't want to do math; i.e, sorting fractional drill bits that have gotten mixed.  I bought these on a whim when I spotted them on sale at a Big Box store - almost completely due to my experience with the yellow Generals. 

Franklin (Chinese) - These are very nice with only a slight bit of light between the tip and base of the jaw (there's also a tiny sliver of light at the tip that is not rendered in this photo).  These saw the light of day (were used) much more often after my wife purchased me a mini-mill about ten years ago, though their purchase pre-dated that event my many years.  Their construction is more appropriate for measuring metal than plastic. Their action is very smooth. For model work, they are too heavy and unbalanced.

Mitutoyo - These have a dead perfect inside and outside jaw fit.  The action is smoother yet than the Franklin (though with a different feel since there is no gear mechanism associated with the movement). They turned out to be more useable for modeling than I expected as they are significantly lighter (1.3 oz, 37 g) and better balanced than the Franklins.

I consider a set of calipers indispensable for this hobby.  Consider adding them to your toolbox if you haven't already done so.

Edit: Added the following 12/19/16:

As far as places to source calipers and many other tools in the hobby, in the past couple of years, I have been going to McMaster Carr for both high quality/cost tools and especially for less expensive equivalents.  McMaster Car usually lists these as "Premium" vs "High Grade" (or sometimes, Economy), respectively.  Inexpensive tools don't have to be bad, and I've come to trust McMaster Carr's buyers choices; their selection of inexpensive tools that I've purchased have been of dependably high quality.  If I know my product, I may shop around, if I don't, I'll buy from MC.  

For calipers, a good place to start:
https://www.mcmaster.com/mv1481823214/#calipers/=15jc26w

Edit: Added the following 10/19/20

As of this writing (10/19/20), the Mitutoyo caliper that was new to me when I wrote this post is still running on the same battery I installed at the end of 2016. This outstanding battery life has virtually eliminated my previous bias for mechanical calipers. I will admit an aesthetic bias toward the spinning dial though...