As mentioned above, there is an issue of getting a clear view of the following traffic in your rear view mirror with the top up. I have the same Sierra top as the OP and have to lower the seat an inch or so from what is normal for top down driving to get a clear view out the back window. It's not a big deal and the sound deadening and overall look of the new top is a huge improvement.
As for access to the engine, it's about he same as with the plastic window original top, so not an issue for me.
The Sierra top comes with DIY instruction and a hotline for questions that are very helpful. If you take it slowly, it is a pretty straightforward installation.
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