I recently switched to a 11-28 (10 speed), I had previously been riding with a 11-25 (10 speed) for over half a year. After using the 11-28 for a month, and all I can say is I'm dumbfounded why more bikes don't have them.
Absolutely love it, I don't feel like my riding has been impacted in any other way than positively. I'm able to climb more effectively with bigger granny gears in each chain ring. In the big chain ring instead of spinning up hills at 80 cadence, it's now 90. When it starts getting steep, I find the ability to just keep my cadence up more important than shifting in smaller increments those last two gears. The only difference between the 11-25 and 11-28 is the former goes from 21->23->25 and the latter goes from 21->24->28. In other words, those last two gears.
Here's an example that should sum up my experience pretty well. Starting in 50-21, let's say I'm pushing hard at 90 cadence. It starts to get steep and my cadence drops by 10 down to 80, so with the 11-25 cassette I switch to 50-23 and without pedaling harder my cadence goes up to 87.6. I'm already pushing it hard, so I can't really afford to pedal harder without going into the red. Alright, this cadence is good I'll stick with it. Now it gets even steeper and my cadence drops another 10 down to 77.6. I don't prefer this, so I switch to 50-25 and now my cadence goes up to 84.3. I can deal with this, my ideal cadence is 90ish so I suppose I'll just power grind a little at 84.3. No biggie, I'm out of gears in the big chain ring anyways. Now it gets even steeper and my cadence drops by 10 again, down to 74.3. I'm not a big fan of this cadence, it's workable but definitely not ideal. I can either A) bear with it, or B) switch to the small chain ring up front and lose a chunk of my momentum as I deal with the awkward change (and if it's about to flatten out soon, this may not be an option as I'd just need to switch back yet again).
Now let's use roughly the same experience above, but with a 11-28. Starting in 50-21 I'm pushing just at what I can sustain running 90 cadence. It gets steeper, cadence drops by 10 down to 80. I switch to 50-24, cadence goes up to 91.4. Great. It gets harder and now my cadence drops by 10 again down to 81.4. I switch to 50-28, and it shoots up to 95. Good. Now the hill gets even steeper, and my cadence goes down by 10 to 85. I'll stick with this no problem - it's certainly better than 74.3, or having to mess with the small chain ring.
What do I lose with the 11-28? On paper, it's very tempting to say "You don't get as smooth of a shift with those last two gears." In experience, I say "Actually, I lose nothing that I need..." What do you use those last two gears for in the cassette? Climbing, and that's when your cadence tends to fluctuate the most. Makes perfect sense to me to have bigger gaps in those gears to deal with the biggest fluctuations.