Smartphones are great for keeping in touch, be that with friends, family or colleagues. It really doesn't matter what device you have, there are plenty of options available to you.SMS and MMS messaging is great, but sometimes you want something more immediate, you want to have a discussion, and importantly, you don't want to run up charges for messaging, especially if you're roaming.There are a number of data-based solutions and today we're looking at an app that's proved it's worth and has plenty of users. If you've never heard of WhatsApp, it might be something you should consider.WhatsApp\nFormat\nAndroid (version tested), iPhone, BlackBerry, WP7, Symbian\nPrice\nFree\nWhere\nGoogle Play\nWhatsApp is a cross-platform messaging app, which will let you send instant messages to your contacts. In this it is nothing unique: iMessage, BBM, Skype, GTalk, Facebook Messenger and so on, also offer the same sort of functions, but as a standalone app, WhatsApp has a lot going for it.The first important point is that it is cross platform. This means you can, for example, contact people on iPhone or BlackBerry from your Android device. It isn't constrained like device specific services - BlackBerry Messenger or iMessage for example.This makes it great in the real world where the people you know all have a different type of phone. Sure, services like Skype offer a similar experience, but WhatsApp has a lighter footprint: it doesn't use much data and it doesn't draw heavily on the battery either (if the stats from our test HTC One X are typical).Of course, BlackBerry users will argue that BBM is better and perhaps they're right; Apple users are right to like iMessage, but in a world where simplicity in communication makes it easier for everyone, WhatsApp has a very valid place.When it comes to use, we've been testing WhatsApp across devices, but this review is written principally of the Android version.\u00a0As soon as you register your phone number within the app, you'll see those contacts that have WhatsApp. It's great, because you're able to get started without any messing around - as long as you have the right phone numbers for people.\u00a0WhatsApp is a messenger app, so it doesn't go as far as offering voice or video calling, but it will let you send various types of additional content as well as words and the all-essential emoticons.Tapping the attachment icon will let you include pictures or video (either from the Gallery, or captured for the purpose there and then), audio (music or recorded voice), your map location or a contact card. Other apps play in here too, so if you use Google Music, for example, that will be offered as a source of music.As such, there is a lot you can do with it. Of course, sending messages uses very little data, but if you start flicking video back and forth, it's going to cost you unless you're sitting on Wi-Fi, which WhatsApp is happy to do.Everything works smoothly and the messaging process is fast enough to be able to have a coherent conversation, so long as both parties have a data connection. We get the feeling that BBM copes better with lesser reception, which seems to be an inherent BlackBerry trait, but otherwise we're impressed with the performance.You also get nice options, such as groups, so you can have a specific group of friends in one chat, perfect for coordinating people. We also like that you can drop a shortcut for a contact, so if you regularly chat to a particular individual, you can have them with a tap of the finger. Naturally, this being Android, WhatsApp will be integrated into your contacts and sharing options too.You can also broadcast messages, letting you send a message to a selection of people you choose off your contact list, for example, that important "It's a boy!" announcement, or "I'm single again!".Notifications are also nice and customisable on the Android version, so you choose how you are alerted, with different settings for groups, if you want them.WhatsApp is also free and we've found that it has been very stable, with the somewhat cumbersome approach that something like Skype comes with.If you're interested in messaging, coordinating groups, or just staying in touch whilst away from your PC, then WhatsApp is well worth trying out.