At the time of writing, the situation with the CMS Joomla versions is the following. 1.5 has not been being supported for six months, the latest stable version is 2.5.6. It does not make sense to examine intermediate versions 1.6 and 1.7 considering the presence of the latter. Also, a week ago Joomla 3.0 came out. 3.5 to be released by the spring of 2013. What are the criteria to choose a version: current to the date and projected - for the next half a year?

First of all, the choice of Joomla version depends on the fact whether you are building a new website or you already have a Joomla site on one of the older versions. We start with the second case.

There is a site of 1.5 - 1.7. The options are:

1. to leave it as is and wait for 3.5;
2. to convert it to 2.5;
3. to convert it directly to 3.0.

Just note that since version 1.6 an upgrade to 2.5 is painless, extensions are fully compatible. So I would migrate sites built on 1.6 and 1.7 to version 2.5 (about 3.0 - see considerations below). As for the sites on 1.5 - it's not that simple, and there is what to think about. Besides, this is exactly my case.

Let's consider the pros and cons of each of the above options.

To the date of writing my Joomla sites (including this one) have been running the 1.5, and there was not a single problem. Transfer from 1.5 to 2.5 requires large input of labor and time because of engines and extensions incompatibilities. On the other hand, the version 2.5 has many advantages, and it's still needed to walk in step with the times to not to miss it later. According to the statement of The Joomla! Project, the support for 1.5 is still partially implemented, but only in case of security problems, and soon after the release of 3.0 will be stopped completely. As a matter of fact, I had not been ever using the support as long as it was provided in full: there were no problems at all for year and a half. So my choice for now is - to leave everything as it is, but gradually get prepared to the migration to 2.5 - to pick extensions similar to those used if their authors did not bother to upgrade them to the new version, and also to elaborate or derive in the Web an optimal strategy of rapid bulk upgrade sites from Joomla 1.5 to Joomla 2.5. The process is already under way, the results will be published.

If you are creating a new site, it is definitely worth to do it on 2.5. Except perhaps the situation when a webmaster is familiar well with the 1.5, has got some workpieces and does not have time and/or desire to learn new features and look for new extensions - then, of course, 1.5 still has the right to exist. But sooner or later you will have to get off it, so try to find time and desire in advance.

Now these are some considerations about the Third Jumla and expectations of 3.5. Some great features have been appeared in 3.0, such as: Twitter Bootstrap - an excellent framework in order to work with an interface; a new interface for mobile devices - of the front-end, and, that is especially cool, of the administration template; simplified just three steps setup; there are dozen and a half another innovations and improvements. So maybe should we just upgrade to it? No, not to rush. First, 2.5, that has been released not long ago, has the status of Long Term Support (LTS), and will be fully supported until the release of 3.5 and for some time after. Second, the version is pretty raw and untested yet, many extensions are not adapted to. And most importantly - the developers have positioned it as an intermediate to 3.5 and do not recommend to migrate a site from 2.5 to it. The main purpose of the Three-Point-Zero coming out is to provide site builders with an advance check out the advantages of Three-Point-Five and meet its release being fully prepared. They also assert that the upgrade 2.5 to 3.5 will be very easy and painless. Thus, you'd rather wait until the spring and in the meantime test Third Joomla on test sites.

Let's summarize.

1. You have a site on the 2.5 - excellent, it is the best option at this point. As new builds are released - update to the latest (2.5.6 today). At the same time test 3.0 and patiently wait for 3.5.
2. You have a site running 1.6 or 1.7 - do not hesitate to upgrade to 2.5. Next step will be the same as in the point 1.
3. You are still on 1.5 - consider all pros & cons of migrating to 2.5. Considerations "for" took advantage - transfer into 2.5 (how? - it's a separate issue, look for it in the internet for the time being), and then - again, see point 1. If you decided to stay with 1.5 - pick the time to study the features of new versions and keep in mind the need to upgrade to them some day. Suppose you found time/energy/desire/money - utilize them to achieve this goal, do not wait for 3.5 - it will be easy to migrate to it in due time.

Upd: 5 months after writing the article - namely, at the first half of March 2013, I report. One of my sites running Joomla 1.5.22 has been hacked twice recently. Security issues of the old Joomla and my own bungling resulted in a decent headache as an urgent upgrade of half-neglected project to a more secure version. Not rule out that the latest build of Joomla 1.5 has less security problems. Lots of sites are still on it, and it will stay in action for a long time. But I had already worked out a scenario of upgrade to the new version, and don't deal with 1.5 anymore. All other considerations stated in the article are still relevant.

I've been trying to describe in detail the above-mentioned process of migrating from Joomla 1.5 to Joomla 2.5 in the corresponding series of articles that will be published soon.