Sketch is a popular tool for web and app design. It has the best features from Photoshop and Illustrator packed into one without the fluff.
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Pro Active community with a lot of resources
Sketch is hugely popular among designers so there are a lot of well maintained community resources for everything from iPhone frames to iOS/Android UI elements & icons.
Pro "Artboards" are great for working on multiple views simultaneously
Artboards let you easily work on multiple views side by side. Great for having a separate artboard on each page for the various responsive sizes.
Pro Vector based yet pixel aware
You can rescale assets without quality loss, easily export x2 assets for retina designs and design high quality icons & artwork. Being pixel aware lets you set a grid and snap objects to it as well as round to the nearest pixel edge to clean up your layers.
Pro Designed specifically for web and mobile UI design & workflows
Sketch is essentially a version of Photoshop built from the ground up to suit the workflow of web designers.
Pro Makes it easy to export assets
Sketch has a built in exporter that supports PDF, JPG and PNG.
Pro Clean UI
Reactive panels keep the UI for Sketch clean from the sort of clutter the panel system in Photoshop suffers from. The panels in Sketch change based on the object in question, saving you from having to have a multiple separate panels.
Pro Built-in grid system
You can set the square grid to whatever dimensions you like as well as set thicker lines every x blocks. You can also configure the color of the lines to make them as obvious or subtle as you wish and toggle the grid with a keyboard shortcut (crtl+g).
Pro Symbols and shared styles
Sketch lets you re-use the same design in multiple places, with changes synced throughout the various places it is used. You can also create text styles to sync typography changes.
Pro "Mirror" makes it really easy to test designs on multiple devices
Mirror lets you connect your iPhone to sketch and see how your current artboard looks on mobile. It's really useful as you can live check changes which lets you rapidly iterate mobile design.
Pro Easily align layers with smart guides
Holding down alt will show the smart guides that show the distances between any layer you hover over to nearby layers or the edges of the artboard. Very useful for checking your spacing or aligning layers.
Con No Linux version
Sketch is currently only available on Mac, which can make it hard to collaborate if you have teammates using Linux.
Con License based payment model
Sketch has recently decided to cease development of major version (2.0, 3.0, 4.0) with free updates in between, and has switched to a license based subscription model. A yearly license costs $99 and includes the latest version of the Sketch software, plus a year of free updates. After this license expires, you can renew for another year of updates - or continue to use the current software without updates.
Con No Windows support
Con Buggy releases and lack of quality assurance
The fast rate of change means new releases often break files and cause havoc. Duplicate symbols are really bad in the latest releases so you have to keep updating.
Con Very feeble raster tools
You have to go elsewhere if you want something more complex than basic vector masking of raster images.
Con Limited compatibility with Photoshop and Illustrator
Although it is possible to export certain Adobe file types in Sketch, compatibility is mostly hit and miss. For example, some elements seem a little offset in Adobe products.