I have an Alto which I bought in May 2010. It has clocked only 5000 km till now, the reason behind such low usage was some rattling noise from rear, which the MASS solved this year after repeated mails to MUL. As long as Alto is concerned, it is a very reliable hatchback. You can get in and out of tight spot and does not require too much of parking space. The AC is very powerful and it chills the cabin within a matter of seconds, though, the power of the vehicle drops a little because of AC, as it is just an 800 cc.
Regarding the mileage, I got 18 km/L in the city and on the highway 25 km/L, the reason behind which is that I drive sedentarily at a speed of 60-08 km/hr with 30% AC. When I bought the Alto, the very next day had to go to my native Mysore and had a chance to check its stability on highway. At one instance, I had to rev the engine above 100km/hr and it took it even without a hiccup. As it was new and was in run-in period, drove at a sendentary speed of 60-70 km/hr and I got a mileage of 23.5 Km/L with 50% A/C . I was very much impressed by the mileage even before 1st service. After the run-in period, I have revved the engine upto 120 km/hr on NH7, and it was stable at that speed, but have not revved it further.
Regarding the bootspace, the front leg space is good, but you will brush the thighs of the co-passenger while shifting the gears. The rear leg space is a little bit cramped, but not so bothersome. Other than these, there is no issue with the Alto. For a budget of 3-3.5L, it is a good buy.