UFO:ET shoots for X-COM and misses. Thankfully, there's a healthy mod community.
With research it follows the typical X-COM path with the UFOpedia. As you research the tech-tree will open. As you engage new aliens with new technology you'll be able to expand the technology tree. One of the problems with this is that you're likely to hit a point where all the technology is researched and you have to capture an alien commander to continue research. After this point, my research took off at a rate that my manufacturing couldn't keep up with. I went from plasma weapons, to ion weapons, to AKEW weapons before I could even manufacture a heavy plasma rifle. This made much of the technology obsolete before I even had the opportunity to use it.
One of the biggest flaws is that secondary bases are simply intercept bases. You can transfer items to these bases, but about the only things you'll be transferring are aircraft and aircraft equipment. You don't get to place any troops for defense (or as secondary squad) and the defensive structures are woefully inadequate in regards to stopping an alien invasion. This means that if aliens invade your base, your only chance of stopping them is while the UFO is on the global map. Apparently, the developers realized this was a problem and so you have 10 days to send a squad to the base to retake it.
The global map is very reminiscent to X-COM. One of the nice things here is that when you send out your intercept fighters at alien craft it occurs real-time. This allows you to have multiple aircraft pounding on a UFO. And, with the bigger UFOs, you're likely to use 5 or 6 aircraft to take it down. Once a UFO is downed, a terror site is announced, an alien base is found, or you have to retake your own base, you can send out a transport to take you to the tactical battlefield.
Once you're at the tactical battlefield, the first thing you'll notice is that the very dated graphics. The second thing you'll notice is that the sound effects are terrible. But, if you play for awhile you'll notice that the AI is much improved over X-COM. Unlike X-COM, you'll very rarely have to enter the UFO to track aliens down. In fact, even on terror missions it's very rare you'll have to enter a building to engage the enemy.
The biggest problem with the combat is that early on your soldiers couldn't hit the broad side of a barn. This is one of those perplexing issues, often found in tactical games, where you're supposedly recruiting the best of the best and they're anything but. Another issue with the soldiers is their line of sight is terrible, they no periphial vision. Even with these woefully inadequate soldiers, you'll still likely get through the missions. The problem is your pool of soldiers will keep getting smaller and smaller because they're spending so much time in sick bay. Sure, they never die, but they're out for 45 days making them useless.
And, now for the high tilt. Fortunately, there's a healthy mod community that fixed many of the issues with this game. You no longer have to play with immortal soldiers. Other mods have reduced hospital time; allow hiring, firing, and even training of soldiers. Other features include improvements to the AI, reduced manufacturing time, experience points for soldiers who survive the mission, the global map being earth, bounty money for aliens killed, and much more.