after about four years of ownership

well, after owning this unit (two of them) plus two batteries.....
This unit seems to be too light a unit to use if you have many additions hanging on your computer and a high wattage power supply. You might be able to get by but will have a very short run time on this battery. If you loose power it will last long enough to get off the computer and shut down. If you ignore the fact that you have lost power and take no action, You may loose work not saved. There does seem to be a problem with buying battery replacements units that are old or for some other reason do not last. It is a budget unit and performs accordingly. Not the heavy unit expected by some who bought a bargain basement priced unit.
It does seem to handle surges and spikes well and will allow you time to shut down if you act upon your notice given. If you do not respond the software will shut down everything ('Xp' also will do this for you) and you will be fine.
This is a unit for the budget minded user. It does the job with the above limitations. Great little unit for the user who requires light work load on it. Heavier units are available. Has many plug-ins. About half offer battery back up while some offer only sure protection. If you want a budget unit this will do the job. If you need a heavy duty battery backup unit buy the 500 unit.