Q: Experience of photos application

before upgrading it is highly suggested you back up your computer or clone it in the event you decide the upgrade was not in your best interest to begin with and you can return to a point in time where your Computer, the OS and the Applications were better suited to your use

Time Machine:

Use Time Machine to back up or restore your Mac - Apple Support

Carbon Copy Cloner:

http://bombich.com/

1. Upgrading to El Capitan does not mean using Photos. If you have up-to-date versions of iPhoto and Aperture you can continue to use them. Even if they do not work then you still don't have to use Photos, there are other apps you might prefer.

2. Photos is at least as capable as iPhoto, it is nowhere near as capable as Aperture. If you were an aperture user think of a 3rd party app like Lightroom or Capture One etc.

Photos is a new program and has different work flows and a learning curve - Photos is substantially better than iPhoto but no where near as capable as Aperture - iPhoto 9.6.1 will still work under El Captain although you can no longer order print products from it and likewise for Aperture 3.6 although if you read through the Aperture forum you will see some issues with Aperture 3.6 and El Captain

see How Photos handles content and metadata from iPhoto and Aperture - Apple Support  and Photos 1.5 vs iPhoto 9.6.1:  Features and Capabilities

LN

Photos has more advanced editing capabilities than iPhoto, especially with the 99¢ External Editors For Photos from the App Store. This user tip shows how: External Editors in Photos Are Here

Here are some user tips from Photos for Mac  User Tips that will help you  transition from iPhoto to Photos:

Photos 1.5 vs iPhoto 9.6.1:  Features and Capabilities

Moments in Photos are the New Events.

External Editors in Photos Are Here

Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

Photo Editing Extensions for Photos for Mac