If you decide to spend real money to open Hobbit Presents, I feel it is important that you know what you might win, and what the odds are. This post is not intended to discuss the appropriateness of Hobbit Presents in Middle-earth, whether they are a form of gambling, or if they might encourage compulsive behavior. Please use the official discussion thread for that.
These are just my observations from testing that I want to share with you. Please feel free to contribute your own observations or opinions about the actual rewards, their relative value, or the odds of winning them.
If you'd like to refer your friends to this thread in-game, tell them to visit tinyurl.com/hobbitpresents.
These odds are a simple analysis of my observations opening 13,934 Silver Hobbit Presents and 11,039 Gold Hobbit Presents playing a level 85 burglar between May 10 and May 13, 2013 with build 4 of Update 11 on the Bullroarer public test server. This would represent more than $10,000 worth of Mithril Coins on the live servers. (Or if these had been free daily and weekly presents, they would represent more than 38 years of Silver Presents and 212 years of Gold Presents.)
First, a note about gift levels. The rewards you receive from opening a present depend on your character's level. These are grouped into rewards for levels 1-9, 10-19, 20-29, ..., 60-69, 70-79, 80-85. You can win piles of metal scraps that can be sold to a vendor for progressively more coin at each level grouping, similar to skirmish bounties.
You can also win Mathom-hunter's Armour (teal-quality) or Fine Mathom-hunter's Armour (gold-quality) with stats corresponding to level 1, 10, 20, ..., 60, 70, 80. You may win light, medium, or heavy armour pieces depending on your character's class.
Some items can't be won at lower levels. For example, from levels 1-9, you can only win Stat Tomes from Rank I to IV; whereas at levels 80-85, you can win them from Rank I to VIII. You can't win rewards related to legendary items at lower levels, either.
Even though I opened a huge number of presents, there is still a statistically significant margin of error in some cases, since some rewards were observed very rarely. I have grouped together the armour, stat tomes, and stat tomes bundles in this presentation in an attempt to minimize this error.
In the following table, the first group of rewards is only found in Silver Presents. The last group of rewards is only found in Gold Presents.
Turbine can and probably will change the rewards and their odds throughout the lifetime of this system. If there are opportunities to test these changes, I'll attempt to keep this post updated.