Have you ever been in a raid where some loot dropped and you needed to make a quick decision whether it was an upgrade or not? Sometimes it can be difficult to make a snap judgement and you often don't have time to look up a gear list.
Rating Buster has been around for a long time now, but it's still the popular choice to help with gear decisions. I find it really handy if I don't have time (or can't be arsed) to open and check a gear list. It adds a list of stats to the bottom of the item's tooltip, and compares them with what you currently have equipped.
When first installed, the addon tries to guess what stats you need based on your class. For pure magic or pure melee classes, this is straightforward and no setting-tweaks are required. For Druids and the other magic-melee hybrid classes however, you will first be met with a huge list of stats in your tooltip as it tries to compare not only spell stats but physical ones as well.
For example, I have a staff, the Engraved Gargoyle Femur in my bags. If I press Shift while mousing over it I can see what I currently have equipped as well as the item tooltip. Rating Buster will still be visible if I don't hold Shift, but you can set it to only show when you are comparing items like this (click to biggify).
You can see in the stats that it has included Feral Attack Power, as well as physical hit, crit and haste. This is way too much information - I'm a Moonkin/Tree, I don't need to know about physical stuff (treepunches notwithstanding). All I need to do to remove that information is go into the settings with "/ratingbuster win" (anything with win in the command has to be good, right?) and remove those values from the "Stat - Physical" list in the "Stat Summary" menu.
While I was at it I removed all the tanking stats like Defense, Parry, and so on.
The new stat list is much easier to read and evaluate.
From this stat summary I can see that the Lockjaw+Shriveled Heart combination has a lot more spell power than the staff, but the staff has hit on it which I may need. In this case, unless I desperately needed the hit, I don't think it would be worth giving up 185 spell power for. Not sure why I still have it in my bags now, to be honest... *disenchants*
One thing to be careful of is to be mindful of comparing items for offspec. Both Resto and Balance have talents that affect our haste, spell power and crit, but Rating Buster only compares based on the spec you are currently in.
You can also set Rating Buster to ignore enchants and gems, or to always use a certain type of gem in comparing ungemmed items. I prefer to leave the gems ignored, as the gem choice often depends on the socket colour. This does mean I have to mentally add in the gem effects but at least I get a choice as to what gem to consider.
Rating Buster really shows its worth when there are two items that are quite close, and you need to see what the differences are. In the image below, these two items aren't that close in stats, but I'll use to demonstrate anyway. The Scourgelord's Baton (soooo lucky to have picked that one up in a pug!) is better in every way than the Shriveled Heart, but I won't use it for Resto as well as Balance spec because I can't afford to lose the haste on the Heart. If however, I can make up the haste somewhere else on my gear, I'll use it for healing even with the hit on it.
So there you have it. Rating Buster doesn't completely take the place of a ranked gear list, but it can help you see the differences between items and make quick decisions.
Are there any other addons that you use to help you choose gear?