All about location
I recently stayed at the Hilton Garden Inn Downtown Austin with my wife and other family members. First off be careful when selecting which Hitlon you want to stay at, there are 4 all within 10 miles of eachother! There is a standard Hilton around the block, which is more luxurious. The weekend we stayed at the H.G.I. downtown, every other hotel was sold out downtown due to some events.
THE GOOD... 
Location... This hotel is a 1 min walk to 6th street. 6th street is where all the action is downtown. All shops, restaurants, bars, bands, etc are found here. It was nice to not have to utilize cabs on this trip. We walked everywhere. Also, hotel was just off main highway to airport. Just a 12 min drive.
The room... The room was very nice. The bed was amazing, havent slept that well in a long time. The curtains provide complete blockage of any sunlight, which also helps with a great sleep. Everything appeared modern and very clean. 
The staff... From check in to check out, the staff was always happy to help. 
18th floor... The 18th floor has a lounge, bar, restaurant. The brunch is excellent.
THE BAD
Property... I will start with the pool. Its increadbily small and there is no good place to layout. The building will block just about all sunlight. Plus on 4 or 5 people could lay around the pool. The hot tub is more the size of a bathtub. It appears like the hotel wanted to add a pool, just for the sake of saying they had a pool, but didnt have the room. It was drained and closed during our visit even though the weather was near 90. Not sure why.
There is a "riverwalk" area behind the hotel. But its a bit shady and dirty. At night its not lit up very well and its decorated with "fake rock" that is falling apart. The decking was completely covered with pigeion droppings. The entire back side of the hotel appears not to be maintained. Frankly, at night i dont recommend walking that way. 
All in all.... this is a nice hotel, close to the action and the airport. If you want a high end luxury stay downtown Austin, try the Omni, Intercontinental, or the Driskill. If you want a more affordable option to those hotels, then this should satisfy your needs nicely.