But should you switch?

The logistics

Just because you can port your number to Google Voice right now, doesn't mean you should do so immediately. Take a look at your cellphone plan. Do you still have a majority of your 2-year contract left? If so, you'll have to pay early termination fees, because you're taking your number and giving it to another provider—even if that provider isn't strictly a cellphone provider.
Also, when you do port your number, you're going to need a new number for your cellphone. (Cellphones need numbers, and you just gave yours to Google Voice. No, they can't share the same number.) This can be your chance to switch providers as well, so look around and see if there's something you want. Verizon iPhone 4 is an option, but I recommend waiting for the next iPhone in June.

Do you really need to port your number?

I understand if you're some kind of professional that has been giving out your number to people for years, or a real estate agent or birthday clown performer that has advertisements up with your number printed on them, but most of you are not. Unless you have a really cool number that you really must hang on to, ask yourself a simple question. "Can you really not just write an email to everyone on your contact list and tell them you're getting a new number?" If you use Google Voice without porting, you can still hang on to your old number and tell any stragglers about your new fancy GV one.

It's easy to get calls...

Let's say you have ported your number to Google Voice, and your number is 212-OLD-NMBR. You still need a new number for your cellphone, so let's say that's 212-NEW-NMBR. You can then assign a bunch of different phones to ring with someone calls 212-OLD-NMBR, like your 212-NEW-NMBR, and your landline, and your work phone. You can even set these individual phones on a schedule to only ring during work hours and home hours, or add one temporarily, like your parents' phone if you're visiting for the holidays.
So, having those neat screening options and being able to pick up any phone you want to answer a call (even your computer, using Gmail.com), is cool.

...but making calls is weird

If you're on an iPhone and make a call like normal, even if your phone is assigned a Google Voice number, it will still appear to come from the number that belongs to your iPhone. In this case, that's 212-NEW-NMBR, the number you just got and don't want to use! But, you can use the Google Voice app, which will make outgoing calls and texts using your 212-OLD-NMBR. Or, you can save what's called a "routing number" or a "direct access number". Whenever someone texts you at your Google Voice number, Google will assign a distinct new number for them. If you want to text or call this person directly from the native phone or SMS app, use this number instead and delete their old number. It's confusing, but it works.
But, if you have an Android phone, things are much simpler. Because Google lets Android apps have deeper access to the phone, Android's Google Voice app can take care of all this routing and number stuff and make every call seem like it's originating from your 212-OLD-NMBR without you doing any extra work. That's one extra incentive for choosing an Android, if you're a heavy Google Voice user.
Of course this is all very complex and nearly impossible to remember if you're on a landline without the ability to pull up a call log, so you can just use the Google Voice web interface and make calls that way. That's how I'm able to use a batphone without any way to dial out.

What can't you do (or do as well) with Google Voice?

As a whole, Google Voice is great, but there are some things you can't do with it. And there are some drawbacks to using the service full time.
• No MMS using your GV number. If people want to MMS you, they'll have to use the number that belongs to your actual cellphone. It's kind of annoying for people to have to have two numbers for you and remember which one to use for what.
• No Facetime using your GV number. You'll have to give out your bare cellphone number again.
• No more visual voicemail if you have an iPhone. You can technically still use it, but you'll have to rig up your settings so that it's your iPhone eventually taking the voicemail. Otherwise, Google's is fine, and can transcribe your voicemails
• Call latency is worse than you're used to. If you're using Google Voice from a landline, you won't notice, but if you're on a cellphone, you'll feel that your words are getting to the other person slower than usual, causing some awkward pauses/simultaneous talking occasionally
• Another point of failure. Although Google Voice doesn't go down much, it does go down sometimes, leaving you unable to make calls for a few hours. You also occasionally have slowdowns, where your texts are delayed not because of the carrier you're on, but because Google's having some issues.

So should I?

If you're the person who really needs to keep your phone number and are nearing the end of your cellphone contract anyway, yes. Take this chance to start fresh with a new service and a fancy new phone.
If not, you don't need to dive straight into Google Voice just yet. Make a GV account, pick a new number (they even let you type in words to pick one that spells something cool), and start playing around with it. If after a while, you're enjoying it and wish you could only use it with your normal number, you have your answer.