Recs.
Updated
SpecsUpdate
Pros
Pro SVG Support
In the era of "retina" displays, 4k UHD, 5k, and even 8k, Scalable Vector Graphics - independent vector images that can scale to any resolution without any display quality loss - are more important now than ever.
And this tool is quite capable of rendering true SVG output suitable for consumption at any display resolution (not a big bunch of rasterized bits in the document, actual paths, points, etc.).
Pro Decent usability, largely thanks to a proper object-selection mode
Designer offers (and defaults to) a common-sense selection mode that makes working with your drawing much easier. Corel Draw, Inkscape, and others also offer this mode, which only selects objects that are fully enclosed by the selection marquee (instead of selecting everything that it merely touches).
Pro Extended slicing and export possibilities
An object can easily be transformed into a slice that can then be exported in various sizes end formats in 1 go. E.g. Export slice A as PNG 1x, 2x and 3x AND GIF 1x AND SVG.
Pro Focused vector graphics tool
Unlike some design tools, Affinity Designer isn't trying to be all things to all people. It's focused on its main area of expertise: vector graphics. That's not to say you can't use a raster image (think a photo in *.jpeg format for example), but it's not built to do much with that other than using it somewhere amidst the layers and that's about it.
Pro Integrates well with Affinity Photo
These are companion apps & switching between them is built in - Photo is a very powerful raster tool with a feature set close/better to Photoshop, it will also use some Photoshop plugins. This allows you to add-on powerful raster capabilities if you want them - put doesn't force you to.
Pro Sketch Alternative (Great for Mixed OS Teams)
For those working in mixed environments that aren't 100% MacOS, you'll find devoting yourself to Sketch.app brings with it...pain. If this fits the bill for what you need feature-wise and you're in a mixed OS environment, it's a very capable replacement for Sketch.app. Note that it doesn't have all the same features, but then again it doesn't need all the same features. Short of organization differences inside the document you're working on, there shouldn't be anything you can't do with Affinity Designer that you could have with Sketch.
Cons
Con UI design errors
Designer does suffer from some UI defects, which can waste time and even prevent users from accessing basic functionality. One example is the bafflingly bad Fill panel, which offers nonsensical tabs, incorrectly labeled (and unlabeled) controls, and a Gradient panel that's nearly unusable.
Unfortunately, the Affinity team doesn't seem open to suggestions for fixing the UI, no matter how factually and demonstrably users make their case. This raises concerns for the product's future.
Con No Preview for Export Function
When exporting graphics for the web there is no chance to get a preview. The only thing you can do is export multiple formats and compressions and compare the results afterwards. People are asking for this basic feature for four years now, but the developers have different priorities.