Aside from needing to press a button, Alexa works just as you'd expect on the Fire HD 8. It accepts all the same voice commands as the Echo devices, and it supports most of the skills from third-party developers. (As Amazon tells it, some devs require specific devices for their skills, but there aren't many of those around.) And yes, if you've connected smart home devices to another Alexa device, you'll be able to control them from the tablet as well.
Because I can't just shout for Alexa, though, I noticed that I use it differently on the Fire HD 8 than on the Echo. It's easy enough ask about the weather while you're reading an e-book or have it tune into your favorite radio station while you're perusing your favorite site. But if your hands are full while cooking, you can't easily reach over to change the episode of a podcast that you're listening to. I learned pretty quickly that the hands-free aspect of Alexa on the Echo and Echo Dot is a big reason why I warmed up to it so quickly.
I can understand why Amazon isn't letting you use your voice to access Alexa on the Fire HD 8, though. It's just a $90 tablet, and while it's a bit faster than the previous version, battery life remains a major concern. It's simpler to have you manually access Alexa than to have a background service sip battery life while it waits for your command. Heck, it took Apple years before it made Siri completely hands-free.
There is one change for the better when it comes to using Alexa on a tablet: It takes advantage of the screen to display some helpful cards based on what you request. If you ask for the weather today, you'll also get a glimpse at what the temperature looks like for the rest of the week. There's also an experimental feature for existing Echo owners called "voice cast" that will display cards about requests to your Echo on your Fire HD. Unfortunately, it didn't work on my Echo or Echo Dot, but I'll give it a pass for now since it's still in testing. Amazon says that feature should be available to owners of the new Echo Dot within the next few weeks.
Despite not being as useful as it is on Echo devices, it's hard to complain about having Alexa on Amazon's tablets. The company isn't charging any extra for the feature, and it's not stopping it from driving down the cost of its slates either. That being said, I can't wait to see Alexa go completely hands-free on future Amazon tablets (and perhaps on the current devices when they're plugged in and charging).