Is el capitan an update for macbook pro and is it any good??

Yes and yes.

You may need to read up on what to expect in El Capitan, compared to older

Mac OS X; one such item that may or may not affect you, is the newest OS X
does not see iPhoto from previous systems and offers Photos for Mac instead.

This is a new default, and you may or may not be able to use older iPhotos.

However, that said, there are new and different things that may be helpful or

perhaps even good if you had not been set up to use older iPhoto application.

Your computer may be able to use a memory chip RAM upgrade to increase

this capacity, if so the means would be to purchase different capacity RAM

chips and install them; be sure they are from a reputable mac-centric vendor.

Backup your old system and original content to an external drive device; you

could upgrade to the new El Capitian OS X 10.11.4 from an earlier OS X, with

little or no issue; be sure there is adequate free or unused capacity in the hard

disk drive so the newer larger system can function properly.

A quality backup would be able to let you restore the older OS X should you find

there was more likable in the old verses the new system. Best to have options.

In any event...

Good luck & happy computing! 🙂