Happy Picasa user here
I've been really happy with Picasa, but I do use Google services and have an Android phone, so it's a good fit for me. I have never even opened iPhoto on the last two mac laptops I've owned, so admittedly it may be much better now. (The 2010 MBA had limited SSD space, and I'd already transitioned to Picasa on the older machine, so I figured not using iPhoto at all would save me some room. Similarly, I've never touched iPhoto on the 2012 MBA.) But I just really like that Picasa is more of a viewer for the file structure I create manually and doesn't create its own library.
Additionally, I have yet to find a better sync to web option for my albums. I can keep my photos in the folders I create, but set only the starred ones for synching online. I don't have to figure out which ones are or aren't online, or set up different folders for what I've shared. And if I change my mind about a photo, I just toggle the star button on each image and the online version of the folder is updated. With unlimited (web-sized) uploads to Picasaweb, this is a perfect sharing option for family & friends.
This set up even works well for my auto-upload/backup to Photobucket (which is for my full-sized image backups) where you choose which system folders you want the Photobucket desktop app to watch. I'm not sure how/whether this works with an iPhoto library, but since I'm using my own folder structure of images, the Photobucket app can see the images inside and backup in the background. Admittedly, I don't really *like* Photobucket's website, but I have a grandfathered plan for the next 18 months with more than enough space to use it as backup web storage, so it's just another safe keeping holding spot. I have another backup sync storage service that works the same way (but not nearly enough space for my photos, so I only use it for document files), but I like that my set up *could* work for that system too, if I ever decided to upgrade that account.
I don't use any Apple iCloud services, only Google ones, so Picasa is a natural fit for me as a photo viewer plus web sync/share. But it can be used stand-alone as a desktop app only, even without a Google account. I might still use it even if that's all I wanted out of it.