There is some seriously terrible information in this thread.
If you don't actually understand what Google Voice is I suggest you LOOK IT UP before you comment. Otherwise you're just going to confuse people who are reading this thread giving them the wrong information.
Google Voice gives you a phone number. That number is independent of any other phone number you have. On the site you can have that number ring all of your phones if you want. During certain times of the day it can ring just one of those or even go straight to voicemail. You can block callers, you can leave personalized voice mail greetings per person, you can even have one person ring your cell phone all the time, while everyone else goes to your home number. It's very versatile.
It does let you call out from your GV number. You can dial your GV number and it gives you a menu system, one of which is place a call, type in the number and it'll ring the number you dialed. It shows up on their caller id as your GV number.
Note: if this doesn't happen then you haven't added your number to the list of your phones. It prompts you with the menu only if your cell # (or whatever number you're calling from) is linked to your account. How else is it going to know you are you?
You can also ring a number through the webapp, even from your computer on the GV site if you want and it'll ring your phones as well.
In terms of texting. It is text only, no MMS. There are tons of ways around "opening" another app. I get all my texts through my IM client. Which works extraordinarily well. I just tell people I text frequently to use my GV number for texts. If they need to send a picture I have the $5/mo 200 txt plan for such cases. People figure it out, it isn't a big deal. If your friends can't figure it out, time to get new friends because they're dumb as crap. Just tell them to add your GV number as a Pager in their address book, and your cell # as Mobile. Pager for texts, Mobile for calls. Like I said, if they can't figure it out, they're dumb.
Voice Mail transcriptions are fun too.
Now in terms of calling. GV is NOT VOIP UNLESS YOU USE THE WEBAPP. If it rings your phone(s) it is using your minutes. If you dial from your phone it uses minutes. Only through Gmail or GV website while talking THROUGH your computer is it not costing you minutes.
Think of GV as a call forwarding service primarily with the addition of some awesome extras like texting and voicemail transcriptions.