Just got my new MacBook Pro with Aperture pre-installed. Have only been playing with it for a couple of days, and I haven't used any similar applications (apart from iPhoto) for comparison, but my first impressions are good.
It's quite fast-- notably faster than Adobe Camera RAW or Canon DPP for RAW conversion, though Aperture is Universal Binary, and the others are PPC Binaries running through Rosetta at this point. Also very fast browsing though my library and searching.
RAW conversion looks pretty nice. Apparently the 1.0 version was horrible (and I can verify it since they still left the 1.0 algorithm available-- it's awful), but Apple seems to have sorted most of this out. You can still see a bit of mosaicing if you pixel hunt, especially if you have made some harsh adjustments. I'll want to go to Canon DPP or ACR for shots I really want to take care with, but Aperture should be pretty tolerable in most cases. I only wish the noise reduction worked better-- the chroma blur reduction is OK, but it leaves a fair bit of luminance noise and you'd still have to go into Noise Ninja to clean up higher ISO shots.
The Adjustments available are no where near what you have in Photoshop, but that's not really the point of the app (though I suspect Apple will be adding to the battery over time anyway-- would be nice to have some distortion controls-- barrel, pincushion, perspective, etc., and the current spot/patch tool is a bit clunky and could be improved.) I don't nomally do a ton of touchup work, so Aperture will be fine for most shots, where I just need to adjust levels, color balance, sharpen, etc. Aperture will save a lot of hard disk (and time!) by not having to convert everything to PSD or TIFF, and the versioning concept with non-destructive edits is great too-- all the adjustments are light-weight on-the-fly overlays directly on top of the RAW file, so you no longer need to hesitate before trying out a bunch of different treatments, and then you can stack them up and sort for easy organisation.
The Loupe struck me as a bit gimmicky at first, but is very cool, and actually quite useful when you want to check out some detail or general sharpness without opening the whole image (it works directly on thumbnails!)
Integration with Photoshop, when you do need to go there, is "OK"... You can choose Photoshop as your external editor, and then Aperture will convert to PSD or TIFF and open directly in Photoshop (this is a bit slow, but may be due to the Photoshop PPC Binary/Rosetta issue), and then when you save it goes directly into your Aperture Library as a new version. Doesn't seem like you can really mix adjustments from the two apps, but not sure how that would work anyway... If you really want to go Photoshop all the way, then you can export the RAW file, convert and edit in Photoshop, and then save and re-import the PSD/TIFF into Aperture. But at that point it's getting inefficient and you're defeating much of the point of Aperture. Still I could see doing that in special circumstances.
Still haven't quite gotten my head around the whole Library concept. It wigs me out a bit to have a single massive tens-of-GB file (really a package) sitting there which is relatively uninspectable. But I guess if you want the efficient versioning scheme and searching there's no other way of doing it. I suspect once I mentally give up control of file management, and just "trust" the app to do everything, it'll be fine.
Haven't really played with the Light Table or the other layout stuff. Light Table seems potentially useful, but I probably won't be making many books and what not. Won't really use the web stuff either, since I'm on PBase!
There are a few GUI glitches-- like the browser sometimes loses your place and scrolls back to the top when your shuffling within a stack and such. Have also occasionally seen parts of my pictures turn black, and I have to resize the pane slightly or similar to get it back. Not a big deal, but a little annoying... I expect these to continue to get ironed out, too.
All in all seems like a decent workflow and asset management tool-- a lot better than iPhoto, to be sure. Not a fault of Aperture, per se, but I'll be happier in general when Adobe gets its act together and releases a Photoshop Universal Binary-- though apparently that won't happen until they do CS3 next year or some such.
One other tricky point that's not on Aperture, per se. The display on my MBP is very vibrant (and this is even the supposedly not-as-vibrant matte version.) This is great since it really makes photos "pop", but can make it tricky deciding how much contrast and saturation you want in an image. I have already seen how bland things can look on the PC monitors I use at work compared with what I saw at home on the MBP. And I just know that if I try to make prints, they won't come out so nice either. So it seems like it will be tricky to find settings which look good on "normal" monitors or in prints that don't just blow out all the colours on the MBP. If only everything were as vivid as the MBP! :)
Aperture's not cheap, but cheaper than it used to be (they dropped the price by $200 after version 1.0 was so crap), and if you have a Mac, you're probably used to paying the Apple "premium"...
A few recent shots processed only through Aperture: