The new add object tool is now in master for the up-coming 2.90, while this closely follows* the original design task, there are areas we might wish to change or improve on.
* negative/positive axes are needed for some object types, currently snapping is always to geometry - not incremental, preview plane was mentioned too which would be good to have.
This tool has a lot of potential. I just wish it had a faster option to add objects on to the surface of another.
By faster I mean, with just one click and drag and the object scales up and is added with the proper ratio already…
Like this:
And please, if possible, it would be great if this tool could work in sculpt mode too.
A tool for easily drawing new objects is certainly welcome. 
However, I think that the UX could be improved a lot.
I think that the implementation is too simplistic and not very user friendly. It would be much easier to judge the form if the new object was not displayed as a wireframe while being created but as a regular mesh. The base could be drawn as a filled shape to make it easy to distinguish from other objects when creating it onto a face of another mesh (a matter of taste probably). Is it too much work at the moment to change these things because it would require one to implement new mesh creation operators?
The current implementation:
Proposed one (using an addon I’ve been developing):
The creation of a sphere also feels 100% weird in my opinion:
Why not show a sphere from the beginning and start extruding immediately?
No segments drawn for a cylinder?
I also think it definitely should be possible to draw the object onto a face easily. Currently that’s not possible as the orientation of the face is not followed.
Floor could also be a placement option.
I also find it annoying that mouse click is used to place the 3D Cursor. Easy to do that accidentally. It’s already possible to do that using shift + right click (at least when using left click select).
Other remarks:
Here are some of my notes while testing the tools out a bit:
Holding Alt while dragging works as a shortcut toggle the origin (Base vs Center)
I’ve been looking forward to this tool
My suggestions in order of priority
That would be a game changer. 
(Also posted this on blender.chat, but re-posting here for transparency)
Here’s an animated mockup video to better show how the original design doc describes this tool:
In this case it’s set to Surface, and the orientation is set to Normal
Relative to the current implementation, these things needs to be added/changed:
eg. trueSpace had object placement based on normal-world plane, and had a cool two mouse trigger based widget-gizmo for adding bevels, geometry shape iterations and changes for full shape control.
Yeah, we want to work fast, that’s all that matters.
So i’ll ask here again, what will happen to the blueprint tool, discarded now? it has some of those feature & more, should have been combined with this tool.
I’m not sure what the status of the blueprint tool is.
Whatever the case, they’re quite separate. Blueprint tool is an edit-mesh geometry construction tool, where as this is an object placement tool which could eventually be used to place existing assets too.
Oh, I hadn’t seen it in this light. That seems much more useful than just primitives…
I am not a fan of one active tool per primitive shape.
Basically, user is adding an object by defining its bounding box.
That way to add objects is pertinent for most majority of objects whatever shape contained in bounding box is.
That should be possible for Grids, Torus or Monkeys, too.
That should be available to addons of Add Mesh (Bolts from Bolt Factory, Teapot from Extra objects, Shelves from Archimesh) and Add Curve (Helix from Extra Objects, Trees from Sapling Tree Gen) categories.
Except for Force Fields, Speakers, Cameras, Lights and Hair type, a click and drag way to add objects is pertinent.
So, I think that would be more legitimate to have one active tool per type.
For example, for Curves and Texts, tool could be slightly different. First click could define bounding box or text box and second extrusion.
For armature and collections instances, it could be a way to define scale of object, at creation.
Okay, so this tool is not meant for just primitive objects we can add other custom ones…since you spoke about objects placement, the LMB sets the 3D Cursor when you’re in surface mode, not sure what’s the point of that.
I hope then we can have the blueprint tool too as it has all the features the guys are asking for.
Trying this tool, it does make a good impression.
Ruminating about ThinkingPolygons idea. Video shows objects placed and aligned automatically. Would be super useful. But I dont think its a full replacement.
It does not look like a tool which could create a variety of objects, but rather like a tool which places/aligns/scales pre-made objects (not limited to spheres, boxes, cones, cylinders, etc …) which are already there. Could help quickly placing assets.
I like that way of interacting. But it is not too specific those gizmos for each geometry and an excess of work to implement and future? at the end also the user must change parameters in a text box for number of cuts and others
It is the first thing I have thought, when the Origin = Base, it should work like this. How it works now maybe it could be Origin = Corner
The orientation of the objects added to the normal face of the background object is also essential, and direct, without going through the 3DCursor, although it should also be another option.
The option “3D Cursor Plane” is difficult to visualize the plane, perhaps the 3D Cursor should show it. When rotation to the 3D cursor was added I made a proposal at https://blender.community/c/rightclickselect/MBbbbc/
Maybe you could take advantage of something.
So far it works really well. 
Some thoughts on it:
SurfaceMode should have an option to orient the baseplane/workingplane perpendicular to the surfacenormal.
Snapping the size of the mousedragged border to edges and verts by holding a modifier key. Grid snapping would be another related handy feature.
Visible Working Plane
A simple offset value that applies to all add tools in normal align mode and shifts the working plane in normal direction forward or backward.
The value should be visible somewhere when the tool is active and could be set eg wiht up and down and reset to zero with some other key.
If this is not out of scope a simple Auto Merge Option would be nice, to combine it with the object of the hitted surface it was drawn upon. Best also including a subtract submode.
The snapping integration is really nice, its just that the snapping tool focuing on the mousecursor itself cause some problems for the addon tool.
When the baseplane is drawn (first step) all 4 corners should be snappable.( well infact 3 points would be enough)
As the logic of the baseplane drawing lets you draw the diagonal of the baseplane its impossible to snap eg. two boxes exactly next to each other with the same coordinates along their touching edge.
Best case would be if the edges of the drawn boundingbox would also snap to other exisiting geometry.
