SpaceCadet

I need to know if its worth it to upgrade the factory non-ML speakers. If so, please provide some brand and models.

rajeev6

I have 2010 HS250h - non-ML but "Premium audio" but nothing was premium with that audio!
I'm assuming that speaker should be similar in IS250 due to similar shape and size of cars.

It was hard to find reliable information from anywhere as the model was very new at that time. I ended up paying $15 for a 2 day subscription at techinfo.toyota.com.

My HS had 1" tweeters in dash, 3.5" center, 6.5" on front doors, 6.5" components on rear doors and an 8" dual voice coil sub on the rear deck.
So far, I've replaced both the front and rear door speakers only - I used JBL components and it made huge difference.
Just a disclaimer : I'm biased towards JBL products - have many at home. I like their speakers for their high efficiency ( > 90db) and I like the bright sound.
If you've another brand in mind, my only recommendation would be to look for high efficiency speakers.

Finally, to your question - "Is it worth it" - it depends upon your values -

1. How important is quality of sound for you?
   Can you tell the difference between Sony receiver and Harman Kardon (or Onkyo/Denon/Marantz) receiver (I'm speaking from my own experience). If you can, I'd say go for it.
2. Money - I spent more than a grand to get good sound (some of them are yet to be installed). 2 sets of components - the decent ones will cost you a minimum of $150. Installation will cost you another probably $150. You may be able to save on installation if you're handy. (Let me know - I can send you directions - for HS though)
3. Time - required to get all this done

Good Luck.