The iPhone app
Simply divided into three sections of Inbox, Greetings and Settings, the YouMail app is a free download. Under Messages you'll find a few folders: Inbox, Save, Spam and Trash. I wasn't able to add folders from within the app, but you can add them on the Web.
From the Inbox you can see who called and when, plus, if you turn on transcription, you'll get a very brief text summary of the message. Tapping into a message again gives you some contact info (you'll see a picture of the contact if you upload your contacts from the app or the site), the text transcription, if you have that enabled, and buttons to reply, move the message (into another folder), play, turn on speaker phone, or delete. Pretty basic stuff, although if you are in a trial period for paid transcription, you'll get a little nag button to upgrade to the paid service (more on this in a moment).
Under greetings you can quickly change your incoming greeting to one of several pre-recorded greetings. I like that you can fake the "number disconnected" greeting, or greet callers with their first name. But there are many more useful greetings besides those. Plus, from within the app itself you can always record and name a new greeting. Mix it up!
The settings for the app allow you to toggle Bluetooth audio output, toggle autoplay messages (although it isn't entirely clear what messages you're toggling), and enable email or SMS alerts to missed calls or voicemails. There's a handy button to use your iPhone's address book and upload all your contacts as well.
Perhaps the best thing about the app is that it'll work on iPad, iPod touch or your iPhone, so even if you don't make calls from your iPad, you can still check your voicemail. And that's the beauty of YouMail overall -- you aren't tied to your phone to check or manage your voicemails. But again, how is this different than Google Voice?
The YouMail site
Google Voice does let you route calls into its voicemail system, so a caller needn't call a different number, they just call your normal cell number, Google "hijacks" the call, and it goes to Google Voice. Plus, GV allows you to keep your messages indefinitely. YouMail allows you to keep 100 of your previous messages under the free account plan. GV does transcription, but only automated (computer) transcriptions, which are pretty hilarious. YouMail offers a paid human-based transcription service. Is it better? A couple of trials and I can say that while it is superior, it is not perfect. The service still misunderstands names it is unfamiliar with, even your own (I was called Beck) and little can help a fast-talker or mumbler or a bad connection.
However, Google Voice has a very limited feature set within the voicemail section. The bare-bones approach may appeal to some, but I will say that YouMail is like Google Voice on steroids when it comes to voicemail. Just under the "More Actions" menu alone, YouMail allows you to forward the message, enable sharing, flag for follow-up, edit the contact, add the caller as a contact or download a bunch of messages in bulk. Forwarding the message alone is worth it, as you can send to anyone via SMS or email (you're not locked into a carrier). With Google Voice you can email, download or embed the message.
With Google Voice you can have personalized greetings. But again, YouMail has more available options and content. For example, there's an entire community of uploaders making greetings you can use! Here's one with a Valentine's Day message in it, but there are quite a few available from the humorous (Rick Roll) to the practical (Do Not Call Registry). Just like Google Voice, you can set these per incoming phone number so that everyone can have their own message. Something where Google might have an advantage is if you use Google groups in your contacts, as you can set messages for those within Google Voice. But I'm guessing there's not a huge market already using their groups feature.
There's some nice granular control available on YouMail for several features, but I think it's worth noting a couple that differentiate YouMail from Google Voice. In YouMail, you can get a push notification either when you receive a voicemail, or after it has been transcribed. You can customize email so that it is tuned for the iPhone, full HTML or just plain text. You can get a text when a caller is ditched, or when someone hangs up on your message. You can get an email with an MP3 of the voicemail messages you've been left.
Then there are two things YouMail did to make voicemail "fun." On the site there's an "Accomplishments" page, which is basically like a list of achievements. If you haven't, say, uploaded a user thumbnail pic, it'll say "Not Complete" and provide a handy link to show you how to do this. A nice feature, although I think YouMail could do a better job of promoting these besides burying them in your user info. Then there's a way to hook into Facebook. Obviously you won't go posting every voicemail you get, but if you happen to get a funny voicemail, and the other party doesn't object, you can actually post it to your Facebook wall. I don't know if that's smart or crazy, but it's there and definitely something Google Voice does not do.
As stated earlier, human-powered transcription will cost you. You can go here to review the plans, but you've got some options for transcription, no ads (yes, the free version has ads), phone support, longer incoming message lengths, and up to 5,000 voicemails can be stored. You have to buy transcription separately, but if you are a power user, you may wish to consider the Pro annual service, costing $19.99 a year (not shown below as it does not include transcription). With this plan, blocked or unknown callers have to input their phone numbers -- that's probably enough for me to pay for it. The Read-It plans include the Pro features but add transcription, a service YouMail pushes heavily. The cheapest transcribes the first 30 seconds of 20 messages per month for just $4.99 a month -- probably enough for most casual users. Unlimited is $34.99 a month.
Is YouMail right for you? If you get a lot of voicemail, it's probably worth a look. Is it worth paying for transcription versus Google Voice? That depends on how much you rely upon voicemail, I think. YouMail is offering a service, and it has a value. I think it's worth testing the service (and they let you try it out without even entering a credit card number) to see if you'll get enough use to justify the cost. It is a decent service, and the features YouMail provides (both free and paid) outdo Google Voice in terms of feature set and content -- never mind the iPhone's built-in voicemail service, which is hardly any comparison.