[General Topics] Menneisyys:
User control: in here, I've elaborated on what buddylist features the reviewed IM client has - for example, is it able to answer to other users' contact addition requests. I've also mentioned if a particular client has user group editing operations (they do support all groups already available on the server side). User groups are very useful; for example, they allow for separating your workmates from your friends.

Editing functionality includes, for example, creating a user group and moving contacts to there. This will, then, be synchronized back to the IM server so that, no matter where you log in from, you'll see the same user group structure in your IM application.

In general, all apps work flawlessly with contacts and groups. The only exception, as of version 0.10, is OctroTalk, which always makes the user have to let all past MSN users added as contacts (also see this AximSite post for more info). This is really annoying!

Voice chat?: some IM applications also support voice chatting (Voice over IP, VoIP). In here, I've elaborated on these features.

File transfer?: file transfer is supported by all instant messaging protocols but, unfortunately, few Windows Mobile clients.

Some apps that do support it (to some degree) do it in a non-direct way; that is, uploading the files to their central server and only pasting the (temporary) URL to the file to the target of the file. This is why you'll only see URL's passed with these clients, not the standard, “embedded” file transfer interface.

Text formatting (AIM : full formatting; Yahoo: Bold / Italic / Underlined; IRC: the same + colors + inverse)? (MSN: Edit / Change font only changes the font of the entire current / following messages; that is, it offers no real formatting capabilities; ICQ and Google Talk: absolutely no formatting capabilities): some IM services / clients, most importantly, Yahoo and IRC (and to a much less degree, MSN), allow for text styling / formatting. In here, I've scrutinized whether Yahoo / IRC / (MSN)-compliant Windows Mobile IM clients are able to correctly render styled text. (None of them are able to actually produce styled text.)

Quick edit shortcuts (Ctrl-A, Delete, Ctrl+arrow etc): Particularly if you use your IM client with an external (for example, Bluetooth) keyboard or use your desktop's keyboard via a Windows Mobile controller application (for example, Pocket Controller – see this for more info), you will want to go for a client that allows for the standard keyboard cursor movement shortcuts to greatly speed up for example text modification / correction before submitting it. Fortunately, only one application (WebMessenger) doesn't support this at all; all the other IM clients support this almost flawlessly.

Bandwidth usage (transmitted/received bytes in kilobytes): login, 10 minutes and a long-time test with one-hour long data: In here, you can not only check the bandwidth usage of most clients but also that of the protocols themselves, should you want to base your service / protocol selection based on (also) bandwidth usage.

If you use a non-unlimited data plan with your data connection-based client, you may also want to know which client and which service / protocol consumes the least bandwidth and what combinations (for example, ICQ with enabled and, in most cases, useless "keep alive" pinging every half a minute) should be entirely avoided. As can be seen, MSN (and Live) Messenger (which both use direct connections) use a bit more bandwidth than most other clients using the same MSN protocol. It's also worth pointing out that the bandwidth usage of Jabber, ICQ and IRC is even less. That is, if you really want to minimize your bandwidth usage, you may want to choose the last three protocols instead of, say, MSN. (Or, if your enterprise already supports it, go straight for Exchange-based Push Mail for notification purposes – particularly after applying the well-known "heartbeat" hack, it consumes the least bandwidth.)

Of particular interest are the bandwidth usage figures of OctroTalk. While its Google Talk (that is, Jabber) and ICQ (in the ICQ test, it consumed a little over the mChat / gsICQ figures) bandwidth usage figures are only slightly smaller than those of the alternative clients / services, the MSN test produced some astonishing results. Compared to the Microsoft MSN / Live Messenger clients, which use about 28 (12+16) kbytes an hour, OctroTalk only uses a little less than 6 (0.5+5.3) kbytes. This means OctroTalk only uses one-fifth (!) of the bandwidth of Microsoft’s official solution. Compared to other MSN-capable clients (all (except for IM+) alternative MSN clients use about half of the uplink bandwidth of the Microsoft clients), it still has some advantage – it uses about 3.5 times less bandwidth to keep the connection up.

This means if you really need to use MSN but you must have the absolutely least possible bandwidth usage, you may want to take a serious look at the OctroTalk client. (Otherwise, if you don’t need to minimize the bandwidth usage, for strictly MSN communication, my personal pick is still Microsoft’s Live Messenger because of the excellent features.)

(MSN) avatars? : Finally, none of the otherwise MSN-compatible clients support MSN avatars (small icons), except for, surprise surprise, Live Messenger. The latter even allow for changing it yourself on your mobile device.

2.2 Not tested

2.2.1 Mobile (GPRS etc.) connection keep-up

In my tests, most applications behaved quite OK. However, there may be problems in your particular case, particularly if you use instant messaging on a mobile phone. In these cases, if you lose the connection after some time, you may want to check out for example this registry hack to fix the problem.

2.2.2 Task keep-alive

I haven’t checked whether the current versions of the IM apps can force the operating system not to shut down the given application when the standard “let’s kill a background” task to free up some memory / when we’ve reached the 32-process limit of all WindowsCE versions (that is, even Windows Mobile 6) before 6 (note that WM6 is still based on the, process number restriction-wise, “old” WinCE 5.2). You won’t have this problem at all if you don’t run memory-hungry and/or several other processes (for example, you don’t open up more than 15-20 tabs in the excellent Windows Mobile Web browser, Opera Mobile). If you only have 4-10 apps running at a time and you have at least 5 Mbytes of free (dynamic) RAM, I’m pretty sure your IM app won’t be silently killed, even when it doesn’t support forcing itself to remain active.

I’ve only made tests with starting all the clients at once but, to avoid clashes, not logging into anywhere. Then, I’ve increased memory usage and the number of active processes by mass-starting other applications. In general, all the tested applications were shut down after a while. Note that IM+ has a checkbox to avoid this situation; I haven't enabled it when running this test.

Please see for example this thread for more info on this problem.

3. Other links of interest

A cool, recommended overview & comparison of some apps

eWeek: "Mobile IM Landscape Shows Room for Growth"

What is your messaging application of choice?

Generic “Looking for a IM / chat client similar to Yahoo messenger“

Gerneric “The Best Instant Messenger (IM)”

Google Talk On Your Pocket PC

Mobile Chat Rooms?

A Jabber-related thread (“Jabber Releases PocketPC Client”)