I'm on my second Android.
On my first, the original Moto Droid, there was no way to Reply All, nor was there a way to even indicate that you were even looking at a group text. I had this phone for 2 years and used both the stock messaging app and handcent.
Then, I upgraded to the Bionic and started right in using Handcent. With handcent, the "all threads" window would show that sender's name, followed by the phone number of every recipient. Even if one of those recipients was a contact in my phone, it would just show the number. Opening the thread and replying would reply to all...which has nearly gotten me in trouble a couple of times! There is no option to reply only to the sender. The only option Handcent has with regard to a reply all is to send them as split threads. In that, each reply is sent individually to each recipient in a different thread. So, it breaks up the message and sends it directly 10 recipients, not as a group text....and if any of them wanted to "reply all" to your recent message, they can't. They can't even see who you sent it to. Pretty stupid, if you ask me.
I just took a look again at my stock messaging app. When I go into a group text thread, I have the option to "reply all" by just typing in the "compose message" box at the bottom of the screen. If I long push on a message within that thread, the last option I have is "reply to sender", which breaks the message out of the current thread and allows you only to reply to the person that sent you the message.
Unfortunately, I'm not a fan of the stock messaging app. My biggest gripe is that when choosing a recipient while composing a message in the stock app....you get EVERY single phone number...home, work, cell, etc. I'm a huge fan of Handcent's "filter mobile contacts" option.....which filters out your contacts when composing a new message to include ONLY cell numbers. After all, who the #$%#7 wants to send a text to a landline??!!??!! I also don't like how the stock apps try to mix all of your messaging together....email, text, facebook, etc. It's a great theory to have a one stop spot for all message. But, if I'm working, I don't really need to be looking at texts, facebook message, etc etc etc!!!
I wish Handcent had a better solution, because I really don't want to go back to the stock app.