More Observations: Once I got all my photos uploaded to iCloud I thought my troubles were over. However I am finding that not to be the case. Here's what I have found with iCloud storage of photos and with the computer software Photos.
On iCloud .....
On iCloud I have, as best I can tell, all 28,000 of my photos plus 8 short videos uploaded. In the section called iCloud > Photos > Moments -- all 28K photos are arranged by date the photo was taken (hence the name "moments" perhaps). Some (very few) are labeled also with the location where the photo was taken (iPhone photos take near my router are labeled with a location!). NOTE: Individual or groups of photos taken on the same day are not labeled with the album names as in the old iPhoto. In iCloud > Photos > Albums it appears that all 315 albums I made in iPhoto are there, but there are a LOT of empty tiles that are labeled simply with dates (the dates I believe are from the "Moments" mentioned above). At this time I see no way to delete the empty tiles. In iCloud > Photos > Albums there are 4 selections you can make, 1.) All Photos (mine appear to be all there), 2.) Favorites (not your Flagged photos from iPhoto - the "flag" designation appears to have been left out of the new software). Favorites are photos you selected from Moments and designated a "heart" icon applied in iCloud or Photos. 3.) Panoramas (unusually long photos - may just be cropped that way). And 4.) Videos.
BOTTOM LINE: Only real unusual thing is the empty tiles in "My Albums" -- and no way to delete them in iCloud.
In the Photos software ......
In PHOTOS you have 4 selections at the top: 1) Photos, 2) Shared, 3) Albums and 4) Projects. I'm going to start with the WORST news, in Photos > Albums I should have 315 albums transferred from iPhoto. I have tiles for 350. None of them have the names I gave them, they only have dates (except for a few with a date and then the words "Photo Stream."). Only 46 have actual photos in them, they do NOT have all the photos in them that were in my original iPhoto albums (not even close)!! Most "albums" here are simply single photos that I photoshopped and just threw back into iPhoto later. Albums is a DISASTER. Apparently, all the work I have put into organizing my 28,000 photos into albums was wasted on my computer ... but NOT in iCloud! (see above).
In Photos > Projects should be all the hardcover / softcover books you made and had Apple print. In Photos I only have books going back 2 years (15 tiles out of perhaps 40 books that I have made), BUT only 4 of those have any photos in them. Thus, only 4 of the 40 books I made seem to have survived the transfer from iPhoto to Photos.
In Photos > Shared are photos you shared like on Facebook. I've shared tons, only 2 show up in this section, one from February 10th of this year, the other from February 20, 2013!? I don't really care about this section, but it's not correct either.
In Photos > Photos, all 28K of my photos appear to be there, they are arranged by DATE photo was taken, not by album name as in iPhoto. Most, but not all, of the dates appear to be correct. 93 photos are dated 2017, 2018 or 2019. No idea why those dates appear. You can chose photos to play a slideshow, "create or add to an album, slideshow or print album," or "share selected photos" like on Facebook. Slideshow works. I did not try sharing -- I'm not confident enough in the software to try sharing a disaster like this on Facebook. Create or Add allow you to add selected photos to an album. You can add it to a "New Album" which you can name, or you can SUPPOSEDLY add it to one of your previously made albums. HOWEVER, as I stated above, only 46 of 315 albums were transferred to Photos and NONE of them were labeled with the correct name (only a date). HOWEVER, when you select Add to an album, you get a dropdown box where I can select one of my 46 NAMED ALBUMS (remember that in the Albums section of this software, NONE are named, plus I am supposed to have 315 albums, not 46). So transferring individual photos to albums is messed up, because you have no album names to work with. (NOTE: You can change the date on a tile to an album name, but that new name does NOT SHOW UP on the dropdown box of album names in Photos > Photos. Albums is a real mess.
I haven't even looked at EDITING the photos yet. I'm leery of making any changes and having Photos upload incorrect data to what will be stored on iCloud.
Bottom Line ---> Albums is #$%&%!#. Albums did not make it correctly from iPhoto to Photos and you cannot even fix it in Photos to get back to where you were in iPhoto (with 28K photos I'm not sure I'd want to ... but it would be nice to have that option, or, better yet, if the **** thing worked correctly in the first place). Worse yet, Albums in iCloud is "almost" okay, but what's going to happen once I start adding new albums to Photos, what data will it send to iCloud? Photos doesn't have the same data for albums as is found in iCloud!!
One does have to wonder how Apple missed all these mistakes. One also has to wonder if they can come up with a giant fix?? It's no wonder they haven't said anything yet. They've put 10.10.3 out for upload ... we first downloaders apparently are the beta testers. One can only hope they can repair the damage. I'm going to leave Photos alone for a while and cross my fingers.
Sorry -- one other kind of unrelated event: Tonight when I looked at my iPhone > Photos I was greeted with the warning: NOT ENOUGH DEVICE STORAGE. This device does not have enough storage space to store all you full resolution photos and videos. To free up space, store device-optimized versions and keep full-resolution copies in iCloud." I was then given the chance to "optimize iPhone Storage" or "Ignore." I optimized. You can select this in SETTINGS > PHOTOS & CAMERA. I have the iPhone 6 with 124GB of memory. After resetting to optimized version I ended up with about 54GB of photos and 36 GB of available memory (storing 28K photos and 8 tiny videos).
Dare I say this ... perhaps those of you that haven't gotten everything up to iCloud yet are ... lucky?!