Distant Suns draws deep-sky objects as colored dots: green for clusters (both open and globular), red and blue for nebulae, and yellow for galaxies. When you zoom in to a field less than 20°, the dots are replaced by Messier numbers. No non-Messier deep-sky objects are included.
To obtain further information about a deep-sky object you can center it in the cross-hairs. Tapping the "..." button then takes you to a detailed information page. (You can also reach this page from the "Other" button in the toolbar, which takes you to a list of all the Messier objects.) As usual, Distant Suns provides far more information than the other apps. There's also a photo of each Messier object.
(The off-center header lines in this image are due to an apparent bug.)
GoSkyWatch includes data on 219 deep-sky objects: the Messier catalog and the so-called "Caldwell catalog" of prominent DSO's that Messier missed. Together, these catalogs include plenty of objects for all but the most serious of amateur observers; the only notable absences are the Magellanic Clouds. Each type of DSO is represented by a different colored symbol. You can hide DSO's that are fainter than the current magnitude limit setting if desired. The information screens include photos of each object and plenty of data.
iStellar shows deep-sky objects as little colored symbols, distinguishing their various types both by symbol and by color. You can set the app to show any combination of symbols, names, and Messier numbers. No non-Messier deep-sky objects are included, and no further information about the Messier objects is provided.
Starmap Pro includes the complete NGC and IC catalogs of deep-sky objects--more than 13,200 in total. The fainter ones appear as you zoom in, just as for stars. You can switch each class of objects on or off in the Settings, but you can't set a magnitude limit for DSOs.
Besides the various symbols for deep-sky objects, Starmap Pro will show circles representing their approximate sizes (when this feature is enabled). You can tap on a DSO to identify it and get ephemeris information, just as for a star or planet. Starmap Pro will also show a small photo in the popup for each of the Messier objects (and for planets).
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Star Walk draws each of the Messier objects as an actual photographic image. You can identify them as you would a star or planet, tapping once to see the Messier number and then tapping the "i" button to see a full-screen image and some data (plus a Wikipedia link).
Unfortunately, the images of Messier objects are not plotted to a consistent scale. Some of the globular cluster are drawn so small that they're almost indistinguishable from stars, while others are magnified to appear proportionally too big and bright. (For instance, M28 incorrectly appears bigger and brighter than M22.) Another glitch causes many of the galaxy distances to be too low by a factor of 10.
Uranus includes a great selection of 500 deep-sky objects, represented as traditional star atlas symbols. The names/numbers disappear when you zoom out, and can also be dimmed using the same dimmer control (hidden at the top of the screen) that dims the constellation lines and boundaries. Tapping on a deep-sky object shows the popup at the bottom of the screen, with the coordinates and magnitude.