I read a free version of the poem. My rating is for the poem alone.
When looking at reviews of this poem, one of the first things you'll notice is that there are a lot of 3 star and 1 star reviews. I think many of the 3 star reviews are from people who hated reading the poem, but feared scorn from their academic peers if they were totally honest. The other group hated the poem, and didn't care what others thought. I'm sure there are plenty of high school students in the latter group.
I love the poem, but I believe the real reasons for people not "getting it" are two-fold. The first is that Tennyson's poetry is full of words and uses of words that we are unfamiliar with today. It's generally not fun to read something if you have to grab the dictionary every other line. The second reason is you must have experienced both deep friendship and loss for Tennyson's elegy to truly resonate with you. Otherwise, you are merely a spectator to another man's grief for over a hundred pages.
Thoreau said that we must put as much effort into reading as the author did into writing, and while I doubt that is possible for this poem, there are some things that can be done to better appreciate it.
1. Slow down-this is the most important step. It's easy to skim when reading poetry, but you'll quickly find that you don't know what the last 3 pages meant. If you have to, read out loud or at least move your lips. Poetry was meant to be heard as well as read.
2. Review-Tennyson broke his poem up into several smaller poems. At the end of each section, pause and make sure you can summarize at least some of what he said (don't expect to get it all.) If you can't, then reread. Most of the mini-poems are 4 to 5 stanzas, at most. In addition, they each tend to focus on a particular theme or perspective. Understanding by chunks will make it easier to see how everything fits into the whole.
3. Take breaks-your mind can only handle so much concentration at once.
If you do these steps I think you will find that In Memoriam is not only a poetic masterpiece, but is also a rather deep philosophical treatise on the meaning of life, memory, evolution, faith and grief.