I used these too for my babies, it is healthier then plastic. It is also easier to clean and you can heat up contents in microwave without feeling like your poisoning your child with plastic chemicals. They are heavier but I've never had a problem with baby holding it. I do not trust plastic bottles and they are harder to really sanitize. I would never use anything other than these for my precious babies.

I have Born Free glass bottles in 9oz and 5oz and I love it! Even though plastic bottles say they are BPA free, I just wanted to be completely free from them. That's why I decided to buy glass bottles instead of plastic ones.I love their venting system too, it bothers baby less as it reduces vacuum building up in the bottle.
The price of the glass bottles are same as plastic bottles, and I could buy these much cheaper when Toys-R-Us were having sales. They sometimes have really good sale price on feeding bottles.
There is a little bit of weight on this, but it's worth it, as it's more durable, easier to wash, keeps milk warm longer than plastic ones.