The Shira route approaches Kilimanjaro from the west and then joins the Machame route. Hence everything that has been said about the Machame climb route also applies to the Shira route.
There are several variations to the Shira route. It can be done in six days but most operators also offer a longer version of it. (A really good operator will also time their departure and stagger their camps in a way that avoids the heaviest traffic on the Machame trail.)
That and the added transport cost can make Shira a more expensive option.
The first day on the Shira route is different to other climb routes:
It follows a four wheel drive route. So you either walk on the road for most of the day (not very attractive) or you opt to drive as far as possible.
The latter not only means you skip the first stage of the climb, the rainforest zone. It also means that you catapult your body to a height of over 3500 m/11500 ft without time for proper acclimatization.
If you live near sea level and you only flew into Tanzania the day before, this may hurt.
Overall, Shira has excellent success rates if the schedule involves a night at Karanga Valley (making for a short and easy day before the summit day). However, the good success rate is partly due to the operators on this route being higher level than on the more crowded routes.
Like the Machame route, the Shira route is for people who are confident in their ability to hike in difficult terrain and camp out for extended periods. It has less traffic but it is a more expensive option. You should also be confident about the way you will react to the altitude on the first day.