My opinion after approx. 1 month on eTap on my P5 (sorry for the long write up ...):
I Absolutely Love it. The big difference for me is not so much the wireless part, but the advantage of being able to shift from the base bar as well as the bar-ends. That does not differentiate it from Di2. eTap makes it look very clean though with no wires visible on the FD and RD. I do have the blip box mounted in the (what I think) the cleanest way possible on a P5. There is enough room to place it in the XLAB frame-mounted pocket behind the stem, but then the cables get pulled every time you turn the bar. Another big advantage over Di2 is the fact that the batteries are interchangeable and you can charge them quickly. In 1 month of riding (>1000 RD shifts and >100 FD shifts), I have only recharged them 1x and it was more of a "habit" then need. The FD is working well with the O-Symmetric 52 ring upfront, but I would strongly recommend a chain catcher anyway. I have not tried yet it with a 54-tooth O-Symmetric because of time constraint but might do so soon.
Real-estate:
When you set it up, take your time when identifying the place for everything and do mount your BTA with bottle and GPS before deciding where to put everything else. This way, you know where and how much space you have remaining and can identify the best place and spot to mount everything. I needed to drill a hole in my bottle holder so I could mount it further backwards to make enough space for my GPS. The additional advantage is that also my steering feel has improved since the CoG of a full bottle now falls nicely between the top-cap and the hub of the front-wheel so almost perfectly neutral.
Blips / Blip box:
You don't need the blips and you can only use the blip-box. You then obviously lose the ability to shift from the base bar as well. You can just mount the blip box between your extensions using the provided mount and you are done. As you can see in the picture I reverse mounted the blip box on my P5 to assure the cables don't rub against my storage. To do this, I drilled a 10mm hole in the P5's top cap and fixed the mount with a screw to the top (I think it is a M4 thread that the mount has) on the bottom. The mount also has an adhesive, but why risk fixing it with adhesive alone, if you can also lock it in place with a screw . I first tried mounting it in the right direction (top-side upwards) but that meant drilling the hole for the fixing screw directly between the two top-cap screws. I feared I might weaken the top-cap too much by doing that so I reverse mounted it. I now actually find it more useful this way, since when I'm cleaning my bike, typically I face rearwards anyway so it makes it "normal operation" again. In addition, the RD and FD both have a button that you can use during cleaning your bike. Put it in the big cog when you begin cleaning, and after some work, push it a few times so the RD shifts down to the smaller cog and now clean the big cogs that where hidden by the chain. Everything is now clean without any "shift effort".
Extensions:
I had not cut my extension too short (luckily) because after the removal of the shifters and installation of the blips, I needed to add 2.5cm back to have enough reach to place my hands in comfort. Initially, I wrapped bar-tape over the blips, but I find that a) bar-tape tends to wear faster or rip on the blips, b) you loose some of the feel when shifting. Since then (not shown in the pictures), I have re-wrapped my bar and the blips are just peeking through so I can push them directly without the bar-tape removing some of the feel. Especially in cold conditions, when you ride with gloves, this might be extra helpful.
eTap <-> Garmin:
The eTap pairs nicely with my Garmin Edge 1000 and you get a nice statistic on the total of shifts FD & RD. If you take the time (all of 30 seconds) and identify the ring-setup and cassette setup, you can show this as well, or you can show the ratio on your screen. Ideal for the tech-/ data-geeks. I have not yet been able to show the battery-status, which is the only thing I would actually really wanted. My Garmin has a battery-field, but I'm assuming this is for the Di2 one. Maybe SRAM / Garmin will add this later so you can keep an eye on this before you run out of juice. I have not yet checked if it actually pairs with the 925XT.
Long story short: I love it, and since changing, I have not looked back or thought: "wish I had not done this..."
I hope to pickup my eTap road bike soon (it actually is waiting for me, just have to drive to my LBS ...).
Pictures: