LastRow in spreadsheet - excel

Question

I always see on SO that people use this to find the last row in a column
lRow = .Range("A" & .Rows.Count).End(xlUp).Row
I have always used this function in my projects to find it as I want the last unused row but now that i always see people using End xlUp approach i am wondering why since it only gives that column which may not be correct.
Function GetLastRow(sh As Worksheet) As Long
Dim X As Long
X = sh.UsedRange.Rows.Count 'force excel to recalculate last row
GetLastRow = sh.Cells.SpecialCells(xlLastCell).Row
End Function
Is there any situation where this function fails to give the last row and why it happens?
Some times it happens that it give me a row that could be many lines after the last row of data I assume due to formatting(exported reports generally - I then work my way back up in a loop to remove these rows if fully blank)
What is UsedRange actually considering as used?
I really want a reliable function that I can depend on in the future

Answer 1

Excel tracks cells sparsely: think of it as tracking a row and column pair for each cell that has ANY information (actually its a bit more complex than that but its still a good way of thinking about it). The information can be formatting or data or formulas or ...
So the last used cell is the last entry in the cell tracking table. But the cell tracking table is not reset just by clearing information.
And since Excel 2007 sh.UsedRange.Rows.Count does not (unfortunately) always reset the cell table.
Mostly when you want to find the last used cell you want to find the last cell that actually contains data or formulas rather than the last cell in the cell table.
Using Range.End(xlUp) etc finds the last VISIBLE cell that contains data or formulas, so needs to be used with care if you hide rows or use filtering. And requires looping code when considering multiple columns. But its the fastest of the 3 main methods.
Using Find is usually the most reliable method (but it ignores shapes and comments, and has problems with merged cells and empty pivot tables):
jLastRow = oSht.Cells.Find(What:="*", LookIn:=xlFormulas, SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
jLastCol = oSht.Range("A1:A" & CStr(jLastRow)).EntireRow.Cells.Find(What:="*", LookIn:=xlFormulas, SearchOrder:=xlByColumns, SearchDirection:=xlPrevious).Column

Answer 2

In Excel, if you enter data from A1 to Z100, then the used range goes from A1 to Z100.
If you delete rows 50 to 100, the used range is still A1 to Z100. Just clearing cells does not reduce the used range. Excel will mark the last row/last column where data was entered previously.
UsedRange can give you an unexpected result if you want to find the last populated cell in the spreadsheet, because it takes into account rows and columns that may have been in use previously.
If you want to find the last row in a column, the method with rows.count and end(xlUp) is more accurate, because it considers the current values. UsedRange may be bigger.